feat(mentor): bulk assignment + coalesced emails + team intros on round open

Round-page bulk-assign UI
- Checkboxes on every project row, header select-all, primary-tinted action
  toolbar that appears when 1+ rows are selected with an "Assign mentor…"
  CTA and Clear. Dialog lists the mentor pool with search (name/email/
  country/expertise), load indicator, and a radio picker.
- Always-visible tip strip when nothing is selected explains the bulk flow
  and offers a one-click "Select all N without a mentor" shortcut.
- New tRPC procedure `mentor.bulkAssign({ mentorId, projectIds })` assigns
  one mentor to many projects in a transaction; idempotent on the per-pair
  `(projectId, mentorId)` unique; per-project in-app notifications still
  fire for each team.
- Mutation invalidates listMentoringProjects, getProjectsNeedingMentor,
  getMentoringImportCandidates, getMentorPool, getRoundStats, project.list
  so the page reflects the new state without a refresh.

Coalesced mentor emails
- New `sendMentorBulkAssignmentEmail` (single email listing every newly-
  assigned project + workspace links) used by `mentor.bulkAssign` and
  `mentor.autoAssignBulkForRound`. The previously-silent auto-fill flow
  now emails mentors at the end of the batch, one combined email per
  mentor regardless of how many projects they received.

Team introduction emails when the round opens
- New `sendTeamMentorIntroductionEmail` lists every assigned mentor with
  name + email and a link to the workspace, so teams can reach out
  directly.
- `activateRound` (round-engine) fires the introduction for every project
  in a MENTORING round that has active mentors when the round opens.
- `mentor.assign`, `mentor.bulkAssign`, and `autoAssignBulkForRound` also
  fire the introduction immediately when the project's MENTORING round is
  already ROUND_ACTIVE — so mentors added mid-round still reach the team.
- Idempotency via the new `MentorAssignment.teamIntroducedAt` column
  (migration 20260526114936) — independent from `notificationSentAt` so
  pre-existing mentor-side stamps don't suppress the team-side email.

src/components/admin/round/mentoring-projects-table.tsx

@@ -2,6 +2,7 @@
 
 import { useMemo, useState } from 'react'
 import Link from 'next/link'
+import { toast } from 'sonner'
 import { trpc } from '@/lib/trpc/client'
 import {
   Table,
@@ -15,7 +16,24 @@ import { Badge } from '@/components/ui/badge'
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
 import { Skeleton } from '@/components/ui/skeleton'
-import { Search, UserPlus, ArrowRight, Sparkles } from 'lucide-react'
+import { Checkbox } from '@/components/ui/checkbox'
+import {
+  Dialog,
+  DialogContent,
+  DialogDescription,
+  DialogFooter,
+  DialogHeader,
+  DialogTitle,
+} from '@/components/ui/dialog'
+import {
+  Search,
+  UserPlus,
+  ArrowRight,
+  Sparkles,
+  Loader2,
+  Download,
+  X,
+} from 'lucide-react'
 import { CountryDisplay } from '@/components/shared/country-display'
 
 type Filter = 'all' | 'unassigned' | 'assigned' | 'wants_only'
@@ -23,12 +41,106 @@ type Filter = 'all' | 'unassigned' | 'assigned' | 'wants_only'
 export function MentoringProjectsTable({ roundId }: { roundId: string }) {
   const [search, setSearch] = useState('')
   const [filter, setFilter] = useState<Filter>('all')
+  const [selected, setSelected] = useState<Set<string>>(new Set())
+  const [bulkOpen, setBulkOpen] = useState(false)
+  const [chosenMentorId, setChosenMentorId] = useState<string>('')
+  const [mentorSearch, setMentorSearch] = useState('')
+
+  const utils = trpc.useUtils()
 
   const { data, isLoading } = trpc.round.listMentoringProjects.useQuery(
     { roundId },
     { refetchInterval: 30_000 },
   )
 
+  const { data: importCandidates } =
+    trpc.round.getMentoringImportCandidates.useQuery({ roundId })
+
+  const { data: mentorPool } = trpc.mentor.getMentorPool.useQuery(
+    {},
+    { enabled: bulkOpen },
+  )
+
+  const bulkAssignMutation = trpc.mentor.bulkAssign.useMutation({
+    onSuccess: (result) => {
+      if (result.assignedCount === 0 && result.skippedCount > 0) {
+        toast.info(
+          `No new assignments — the selected mentor is already on all ${result.skippedCount} project${result.skippedCount === 1 ? '' : 's'}.`,
+        )
+      } else {
+        toast.success(
+          `Assigned mentor to ${result.assignedCount} project${
+            result.assignedCount === 1 ? '' : 's'
+          }${result.skippedCount > 0 ? ` (${result.skippedCount} already had this mentor)` : ''}${
+            result.emailSent ? ' · email sent' : ''
+          }`,
+        )
+      }
+      utils.round.listMentoringProjects.invalidate({ roundId })
+      utils.round.getProjectsNeedingMentor.invalidate({ roundId })
+      utils.round.getMentoringImportCandidates.invalidate({ roundId })
+      utils.mentor.getMentorPool.invalidate()
+      utils.mentor.getRoundStats.invalidate({ roundId })
+      utils.project.list.invalidate()
+      setSelected(new Set())
+      setChosenMentorId('')
+      setMentorSearch('')
+      setBulkOpen(false)
+    },
+    onError: (err) => toast.error(err.message),
+  })
+
+  const advanceMutation = trpc.round.advanceProjects.useMutation({
+    onSuccess: (result) => {
+      toast.success(
+        `Imported ${result.advancedCount} project${
+          result.advancedCount === 1 ? '' : 's'
+        } from ${result.targetRoundName ? '' : ''}${
+          importCandidates?.priorRound?.name ?? 'the prior round'
+        }`,
+      )
+      utils.round.listMentoringProjects.invalidate({ roundId })
+      utils.round.getMentoringImportCandidates.invalidate({ roundId })
+      utils.round.getProjectsNeedingMentor.invalidate({ roundId })
+    },
+    onError: (err) => toast.error(err.message),
+  })
+
+  const importBanner = importCandidates?.priorRound &&
+    importCandidates.pendingCount > 0 && (
+      <div className="flex flex-col gap-2 rounded-md border border-amber-300 bg-amber-50 px-4 py-3 text-sm sm:flex-row sm:items-center sm:justify-between">
+        <div className="text-amber-900">
+          <span className="font-medium">
+            {importCandidates.pendingCount} PASSED project
+            {importCandidates.pendingCount === 1 ? '' : 's'}
+          </span>{' '}
+          from{' '}
+          <span className="font-medium">
+            {importCandidates.priorRound.name}
+          </span>{' '}
+          {importCandidates.pendingCount === 1 ? "isn't" : "aren't"} in this
+          mentoring round yet.
+        </div>
+        <Button
+          size="sm"
+          onClick={() =>
+            advanceMutation.mutate({
+              roundId: importCandidates.priorRound!.id,
+              targetRoundId: roundId,
+            })
+          }
+          disabled={advanceMutation.isPending}
+        >
+          {advanceMutation.isPending ? (
+            <Loader2 className="mr-1.5 h-4 w-4 animate-spin" />
+          ) : (
+            <Download className="mr-1.5 h-4 w-4" />
+          )}
+          Import {importCandidates.pendingCount}
+        </Button>
+      </div>
+    )
+
   const filtered = useMemo(() => {
     if (!data) return []
     const q = search.trim().toLowerCase()
@@ -72,8 +184,20 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
 
   if (!data || data.projects.length === 0) {
     return (
-      <div className="rounded-md border bg-muted/30 px-4 py-12 text-center text-sm text-muted-foreground">
-        No projects in this mentoring round yet.
+      <div className="space-y-3">
+        {importBanner}
+        <div className="rounded-md border bg-muted/30 px-4 py-12 text-center text-sm text-muted-foreground">
+          No projects in this mentoring round yet.
+          {!importBanner && (
+            <>
+              {' '}Use{' '}
+              <span className="font-medium text-foreground">
+                Add Project to Round
+              </span>{' '}
+              to populate it.
+            </>
+          )}
+        </div>
       </div>
     )
   }
@@ -103,6 +227,7 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
 
   return (
     <div className="space-y-3">
+      {importBanner}
       <div className="flex flex-col gap-2 sm:flex-row sm:items-center sm:justify-between">
         <div className="flex flex-wrap gap-1.5">
           <Pill value="all" label="All" count={totals.total} />
@@ -121,10 +246,80 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
         </div>
       </div>
 
+      {selected.size > 0 ? (
+        <div className="flex flex-col gap-2 rounded-md border border-primary/30 bg-primary/5 px-4 py-2.5 text-sm sm:flex-row sm:items-center sm:justify-between">
+          <div>
+            <span className="font-medium">{selected.size}</span>{' '}
+            <span className="text-muted-foreground">
+              project{selected.size === 1 ? '' : 's'} selected
+            </span>
+          </div>
+          <div className="flex flex-wrap gap-2">
+            <Button size="sm" onClick={() => setBulkOpen(true)}>
+              <UserPlus className="mr-1.5 h-4 w-4" />
+              Assign mentor…
+            </Button>
+            <Button
+              size="sm"
+              variant="outline"
+              onClick={() => setSelected(new Set())}
+            >
+              <X className="mr-1 h-4 w-4" />
+              Clear
+            </Button>
+          </div>
+        </div>
+      ) : (
+        <div className="flex items-center justify-between rounded-md border border-dashed bg-muted/20 px-4 py-2 text-xs text-muted-foreground">
+          <span>
+            Tip: tick checkboxes to bulk-assign one mentor to multiple
+            projects in a single click (mentor gets one combined email).
+          </span>
+          {totals.unassigned > 0 && (
+            <button
+              type="button"
+              className="text-xs font-medium text-foreground hover:underline"
+              onClick={() => {
+                setFilter('unassigned')
+                setSelected(
+                  new Set(
+                    data.projects
+                      .filter((p) => p.mentors.length === 0)
+                      .map((p) => p.id),
+                  ),
+                )
+              }}
+            >
+              Select all {totals.unassigned} without a mentor
+            </button>
+          )}
+        </div>
+      )}
+
       <div className="overflow-hidden rounded-md border">
         <Table>
           <TableHeader>
             <TableRow>
+              <TableHead className="w-10">
+                <Checkbox
+                  checked={
+                    filtered.length > 0 &&
+                    filtered.every((p) => selected.has(p.id))
+                  }
+                  onCheckedChange={(checked) => {
+                    setSelected((prev) => {
+                      const next = new Set(prev)
+                      if (checked) {
+                        filtered.forEach((p) => next.add(p.id))
+                      } else {
+                        filtered.forEach((p) => next.delete(p.id))
+                      }
+                      return next
+                    })
+                  }}
+                  aria-label="Select all visible"
+                />
+              </TableHead>
               <TableHead>Project</TableHead>
               <TableHead>Wants?</TableHead>
               <TableHead>Mentors</TableHead>
@@ -135,7 +330,7 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
             {filtered.length === 0 ? (
               <TableRow>
                 <TableCell
-                  colSpan={4}
+                  colSpan={5}
                   className="py-8 text-center text-sm text-muted-foreground"
                 >
                   No projects match the current filter.
@@ -143,7 +338,24 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
               </TableRow>
             ) : (
               filtered.map((p) => (
-                <TableRow key={p.id}>
+                <TableRow
+                  key={p.id}
+                  data-state={selected.has(p.id) ? 'selected' : undefined}
+                >
+                  <TableCell>
+                    <Checkbox
+                      checked={selected.has(p.id)}
+                      onCheckedChange={(checked) =>
+                        setSelected((prev) => {
+                          const next = new Set(prev)
+                          if (checked) next.add(p.id)
+                          else next.delete(p.id)
+                          return next
+                        })
+                      }
+                      aria-label={`Select ${p.title}`}
+                    />
+                  </TableCell>
                   <TableCell>
                     <div className="font-medium">{p.title}</div>
                     <div className="text-xs text-muted-foreground">
@@ -224,6 +436,162 @@ export function MentoringProjectsTable({ roundId }: { roundId: string }) {
           </TableBody>
         </Table>
       </div>
+
+      <Dialog
+        open={bulkOpen}
+        onOpenChange={(next) => {
+          if (!next) {
+            setBulkOpen(false)
+            setChosenMentorId('')
+            setMentorSearch('')
+          }
+        }}
+      >
+        <DialogContent>
+          <DialogHeader>
+            <DialogTitle>
+              Assign mentor to {selected.size} project
+              {selected.size === 1 ? '' : 's'}
+            </DialogTitle>
+            <DialogDescription>
+              Choose one mentor — they'll receive a single email listing every
+              new assignment. Projects where they're already an active mentor
+              will be skipped.
+            </DialogDescription>
+          </DialogHeader>
+
+          <div className="space-y-3">
+            <div className="relative">
+              <Search className="absolute left-2.5 top-1/2 h-3.5 w-3.5 -translate-y-1/2 text-muted-foreground" />
+              <Input
+                value={mentorSearch}
+                onChange={(e) => setMentorSearch(e.target.value)}
+                placeholder="Search mentor by name, email, country, or expertise…"
+                className="pl-8"
+              />
+            </div>
+            <div className="max-h-72 overflow-y-auto rounded-md border">
+              {(() => {
+                const mentors = mentorPool?.mentors ?? []
+                const q = mentorSearch.trim().toLowerCase()
+                const filteredMentors = q
+                  ? mentors.filter((m) =>
+                      [
+                        m.name ?? '',
+                        m.email,
+                        m.country ?? '',
+                        ...(m.expertiseTags ?? []),
+                      ]
+                        .join(' ')
+                        .toLowerCase()
+                        .includes(q),
+                    )
+                  : mentors
+                if (mentors.length === 0) {
+                  return (
+                    <p className="p-4 text-center text-sm text-muted-foreground">
+                      No mentors in the pool yet.{' '}
+                      <Link
+                        href="/admin/members?tab=mentors"
+                        className="underline-offset-2 hover:underline"
+                      >
+                        Add mentors
+                      </Link>
+                      .
+                    </p>
+                  )
+                }
+                if (filteredMentors.length === 0) {
+                  return (
+                    <p className="p-4 text-center text-sm text-muted-foreground">
+                      No mentors match &ldquo;{mentorSearch}&rdquo;.
+                    </p>
+                  )
+                }
+                return filteredMentors.map((m) => {
+                  const isChosen = chosenMentorId === m.id
+                  return (
+                    <label
+                      key={m.id}
+                      className={`flex cursor-pointer items-start gap-3 border-b px-3 py-2 text-sm last:border-b-0 ${
+                        isChosen ? 'bg-accent' : 'hover:bg-muted/50'
+                      }`}
+                    >
+                      <input
+                        type="radio"
+                        name="bulk-mentor"
+                        className="mt-1"
+                        checked={isChosen}
+                        onChange={() => setChosenMentorId(m.id)}
+                      />
+                      <div className="min-w-0 flex-1">
+                        <div className="font-medium">
+                          {m.name ?? 'Unnamed'}
+                        </div>
+                        <div className="truncate text-xs text-muted-foreground">
+                          {m.email}
+                          {m.country && <> · {m.country}</>}
+                        </div>
+                        {m.expertiseTags && m.expertiseTags.length > 0 && (
+                          <div className="mt-1 flex flex-wrap gap-1">
+                            {m.expertiseTags.slice(0, 4).map((t) => (
+                              <Badge
+                                key={t}
+                                variant="secondary"
+                                className="text-[10px]"
+                              >
+                                {t}
+                              </Badge>
+                            ))}
+                            {m.expertiseTags.length > 4 && (
+                              <Badge variant="outline" className="text-[10px]">
+                                +{m.expertiseTags.length - 4}
+                              </Badge>
+                            )}
+                          </div>
+                        )}
+                      </div>
+                      <div className="shrink-0 text-right text-xs tabular-nums text-muted-foreground">
+                        {m.currentAssignments}
+                        {m.maxAssignments != null && `/${m.maxAssignments}`}{' '}
+                        load
+                      </div>
+                    </label>
+                  )
+                })
+              })()}
+            </div>
+          </div>
+
+          <DialogFooter>
+            <Button
+              variant="outline"
+              onClick={() => {
+                setBulkOpen(false)
+                setChosenMentorId('')
+                setMentorSearch('')
+              }}
+            >
+              Cancel
+            </Button>
+            <Button
+              onClick={() =>
+                bulkAssignMutation.mutate({
+                  mentorId: chosenMentorId,
+                  projectIds: Array.from(selected),
+                })
+              }
+              disabled={!chosenMentorId || bulkAssignMutation.isPending}
+            >
+              {bulkAssignMutation.isPending && (
+                <Loader2 className="mr-1.5 h-4 w-4 animate-spin" />
+              )}
+              Assign to {selected.size} project
+              {selected.size === 1 ? '' : 's'}
+            </Button>
+          </DialogFooter>
+        </DialogContent>
+      </Dialog>
     </div>
   )
 }