feat(mentor): allow stacking mentors per team; send per-team assignment email

- mentor.assign no longer rejects on existing mentor; rejects only on
  duplicate (projectId, mentorId) via P2002 catch.
- After successful create, sendMentorTeamAssignmentEmail fires once and
  stamps MentorAssignment.notificationSentAt for idempotency.
- All existing behavior preserved: audit log, in-app notifications,
  MENTORING round auto-transition.
- mentor.getSuggestions no longer short-circuits when a mentor is already
  assigned — the suggestions list is now informational and the per-pair
  unique constraint enforces correctness at assign time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/server/routers/mentor.ts

@@ -1,7 +1,8 @@
 import { z } from 'zod'
 import { TRPCError } from '@trpc/server'
 import { router, mentorProcedure, adminProcedure, protectedProcedure } from '../trpc'
-import { MentorAssignmentMethod, type PrismaClient } from '@prisma/client'
+import { MentorAssignmentMethod, Prisma, type PrismaClient } from '@prisma/client'
+import { sendMentorTeamAssignmentEmail } from '@/lib/email'
 import {
   getAIMentorSuggestions,
   getRoundRobinMentor,
@@ -86,16 +87,11 @@ export const mentorRouter = router({
         },
       })
 
-      // TODO(PR8 Task 8): surface all mentors. Legacy single-mentor early-return.
+      // With multi-mentor (PR8) the project can have several mentors. The
+      // suggestions endpoint is informational — return whatever AI suggests
+      // and let `mentor.assign` enforce per-pair uniqueness. We still surface
+      // an existing primary mentor in the payload so UIs can label it.
       const primaryMentor = project.mentorAssignments[0] ?? null
-      if (primaryMentor) {
-        return {
-          currentMentor: primaryMentor,
-          suggestions: [],
-          source: 'ai' as const,
-          message: 'Project already has a mentor assigned',
-        }
-      }
 
       // Detect AI configuration so the UI can label "AI matching unavailable"
       // when we fall back to algorithmic ranking. An AI error mid-call still
@@ -142,7 +138,9 @@ export const mentorRouter = router({
       })
 
       return {
-        currentMentor: null,
+        // TODO(PR8 Task 8): return the full mentor list. Legacy field kept
+        // until the admin UI is updated.
+        currentMentor: primaryMentor,
         suggestions: enrichedSuggestions.filter((s) => s.mentor !== null),
         source,
         message: null,
@@ -221,26 +219,24 @@ export const mentorRouter = router({
       })
     )
     .mutation(async ({ ctx, input }) => {
-      // Verify project exists and doesn't have a mentor
+      // Verify project exists (multi-mentor: stacking is allowed; duplicate
+      // (projectId, mentorId) pairs are rejected by the unique constraint
+      // below).
       const project = await ctx.prisma.project.findUniqueOrThrow({
         where: { id: input.projectId },
-        include: { mentorAssignments: { select: { id: true } } },
       })
 
-      if (project.mentorAssignments.length > 0) {
-        throw new TRPCError({
-          code: 'CONFLICT',
-          message: 'Project already has a mentor assigned',
-        })
-      }
-
       // Verify mentor exists
       const mentor = await ctx.prisma.user.findUniqueOrThrow({
         where: { id: input.mentorId },
       })
 
-      // Create assignment
-      const assignment = await ctx.prisma.mentorAssignment.create({
+      // Create assignment. P2002 on the composite (projectId, mentorId) unique
+      // constraint means this exact mentor is already on this team — surface a
+      // friendly error.
+      let assignment
+      try {
+        assignment = await ctx.prisma.mentorAssignment.create({
           data: {
             projectId: input.projectId,
             mentorId: input.mentorId,
@@ -267,6 +263,18 @@ export const mentorRouter = router({
             },
           },
         })
+      } catch (err) {
+        if (
+          err instanceof Prisma.PrismaClientKnownRequestError &&
+          err.code === 'P2002'
+        ) {
+          throw new TRPCError({
+            code: 'CONFLICT',
+            message: 'This mentor is already assigned to that project.',
+          })
+        }
+        throw err
+      }
 
       // Audit outside transaction so failures don't roll back the assignment
       await logAudit({
@@ -281,6 +289,8 @@ export const mentorRouter = router({
           mentorId: input.mentorId,
           mentorName: assignment.mentor.name,
           method: input.method,
+          // PR8: per-team assignment (one row per mentor-project pair).
+          assignmentScope: 'per-team',
         },
         ipAddress: ctx.ip,
         userAgent: ctx.userAgent,
@@ -322,6 +332,27 @@ export const mentorRouter = router({
         },
       })
 
+      // Send per-team email notification once per assignment row. Idempotency
+      // is enforced via MentorAssignment.notificationSentAt — a fresh row has
+      // it null. If the same mentor is later dropped and re-assigned (new row,
+      // fresh id), a new email is sent — intentional.
+      if (assignment.notificationSentAt == null && assignment.mentor.email) {
+        await sendMentorTeamAssignmentEmail(
+          assignment.mentor.email,
+          assignment.mentor.name,
+          assignment.project.title,
+          input.projectId,
+        )
+        try {
+          await ctx.prisma.mentorAssignment.update({
+            where: { id: assignment.id },
+            data: { notificationSentAt: new Date() },
+          })
+        } catch (e) {
+          console.error('[Mentor] failed to stamp notificationSentAt (non-fatal):', e)
+        }
+      }
+
       // Auto-transition: mark project IN_PROGRESS in any active MENTORING round
       try {
         const mentoringPrs = await ctx.prisma.projectRoundState.findFirst({