feat(admin): bulk role updates + mentor-onboarding email (§D.2-3)

user.bulkUpdateRoles({userIds, addRole?, removeRole?}) batches role
changes across up to 200 users with a SUPER_ADMIN self-demote guard.
When MENTOR is freshly added, fires sendMentorOnboardingEmail once per
user, gated by User.mentorOnboardingSentAt for idempotency. Audit log
entry per user changed.

UI: 'Add MENTOR role' button surfaces in the existing /admin/members
bulk-selection toolbar when ≥1 user is selected. Other roles
(OBSERVER / AWARD_MASTER) supported by the procedure but not yet wired
to UI; one button keeps the toolbar minimal until a clear need arises.

Tests cover happy path, idempotency on second call, removeRole semantics,
and the SUPER_ADMIN self-demote guard.

Plan: docs/superpowers/plans/2026-04-28-pr6-multi-role-and-workspace-previews.md

src/components/admin/members-content.tsx

@@ -191,6 +191,20 @@ export function MembersContent() {
     },
   })
 
+  const bulkUpdateRoles = trpc.user.bulkUpdateRoles.useMutation({
+    onSuccess: (r) => {
+      const parts: string[] = []
+      if (r.updated > 0) parts.push(`Updated ${r.updated} user${r.updated === 1 ? '' : 's'}`)
+      if (r.alreadyHadRole > 0) parts.push(`${r.alreadyHadRole} already had role`)
+      toast.success(parts.join(' · ') || 'No changes')
+      setSelectedIds(new Set())
+      utils.user.list.invalidate()
+    },
+    onError: (error) => {
+      toast.error(error.message || 'Failed to update roles')
+    },
+  })
+
   const selectableUsers = useMemo(
     () => data?.users ?? [],
     [data?.users]
@@ -321,9 +335,29 @@ export function MembersContent() {
           <Card>
             <CardContent className="py-3 flex flex-wrap items-center justify-between gap-2">
               <p className="text-sm text-muted-foreground">
-                Selection persists across pages and filters.
+                {selectedIds.size > 0
+                  ? `${selectedIds.size} selected. Selection persists across pages and filters.`
+                  : 'Selection persists across pages and filters.'}
               </p>
               <div className="flex items-center gap-2">
+                {selectedIds.size > 0 && (
+                  <Button
+                    variant="outline"
+                    size="sm"
+                    onClick={() =>
+                      bulkUpdateRoles.mutate({
+                        userIds: Array.from(selectedIds),
+                        addRole: 'MENTOR',
+                      })
+                    }
+                    disabled={bulkUpdateRoles.isPending}
+                  >
+                    {bulkUpdateRoles.isPending ? (
+                      <Loader2 className="h-4 w-4 mr-2 animate-spin" />
+                    ) : null}
+                    Add MENTOR role
+                  </Button>
+                )}
                 <Button
                   variant="outline"
                   size="sm"