feat(workspace): mentor + applicant message previews (§F.2)

mentor.getRecentMessages: last N unread messages from teams across all
of a mentor's assignments. Drives a Recent Messages card on /mentor.

applicant.getMentorConversationPreview: last 3 messages + unread count
for a given project. Drives a 'Conversation with [Mentor]' card on
/applicant — auto-hides when no mentor is assigned.

Both procedures use the existing MentorMessage(projectId, createdAt)
composite index — no new index needed.

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

tests/unit/workspace-previews.test.ts

@@ -0,0 +1,147 @@
+import { afterAll, describe, expect, it } from 'vitest'
+import { prisma, createCaller } from '../setup'
+import {
+  createTestProgram,
+  createTestProject,
+  cleanupTestData,
+  uid,
+} from '../helpers'
+import { mentorRouter } from '../../src/server/routers/mentor'
+import { applicantRouter } from '../../src/server/routers/applicant'
+import type { UserRole } from '@prisma/client'
+
+async function createUserWithRoles(primaryRole: UserRole, rolesArray: UserRole[]) {
+  const id = uid('user')
+  return prisma.user.create({
+    data: {
+      id,
+      email: `${id}@test.local`,
+      name: `Test ${primaryRole}`,
+      role: primaryRole,
+      roles: rolesArray,
+      status: 'ACTIVE',
+    },
+  })
+}
+
+describe('mentor.getRecentMessages', () => {
+  const programIds: string[] = []
+  const userIds: string[] = []
+
+  afterAll(async () => {
+    for (const programId of programIds) {
+      await prisma.mentorAssignment.deleteMany({ where: { project: { programId } } })
+      await cleanupTestData(programId, [])
+    }
+    if (userIds.length > 0) {
+      await prisma.user.deleteMany({ where: { id: { in: userIds } } })
+    }
+  })
+
+  it('returns recent unread messages from team across mentor assignments', async () => {
+    const program = await createTestProgram({ name: `recent-msgs-${uid()}` })
+    programIds.push(program.id)
+    const mentor = await createUserWithRoles('MENTOR', ['MENTOR'])
+    const applicant = await createUserWithRoles('APPLICANT', ['APPLICANT'])
+    userIds.push(mentor.id, applicant.id)
+    const project = await createTestProject(program.id)
+    const ma = await prisma.mentorAssignment.create({
+      data: {
+        projectId: project.id,
+        mentorId: mentor.id,
+        method: 'MANUAL',
+        workspaceEnabled: true,
+      },
+    })
+    await prisma.mentorMessage.create({
+      data: {
+        projectId: project.id,
+        senderId: applicant.id,
+        message: 'Question 1',
+        workspaceId: ma.id,
+        isRead: false,
+      },
+    })
+    await prisma.mentorMessage.create({
+      data: {
+        projectId: project.id,
+        senderId: applicant.id,
+        message: 'Question 2',
+        workspaceId: ma.id,
+        isRead: false,
+      },
+    })
+    await prisma.mentorMessage.create({
+      data: {
+        projectId: project.id,
+        senderId: applicant.id,
+        message: 'old',
+        workspaceId: ma.id,
+        isRead: true,
+      },
+    })
+
+    const caller = createCaller(mentorRouter, { id: mentor.id, email: mentor.email, role: 'MENTOR' })
+    const result = await caller.getRecentMessages({ limit: 5 })
+
+    expect(result.unread.length).toBe(2)
+    expect(result.unread[0].message).toContain('Question')
+  })
+})
+
+describe('applicant.getMentorConversationPreview', () => {
+  const programIds: string[] = []
+  const userIds: string[] = []
+
+  afterAll(async () => {
+    for (const programId of programIds) {
+      await prisma.mentorAssignment.deleteMany({ where: { project: { programId } } })
+      await cleanupTestData(programId, [])
+    }
+    if (userIds.length > 0) {
+      await prisma.user.deleteMany({ where: { id: { in: userIds } } })
+    }
+  })
+
+  it('returns last 3 messages + unread count for the project', async () => {
+    const program = await createTestProgram({ name: `conv-preview-${uid()}` })
+    programIds.push(program.id)
+    const mentor = await createUserWithRoles('MENTOR', ['MENTOR'])
+    const applicant = await createUserWithRoles('APPLICANT', ['APPLICANT'])
+    userIds.push(mentor.id, applicant.id)
+    const project = await createTestProject(program.id)
+    await prisma.teamMember.create({
+      data: { projectId: project.id, userId: applicant.id, role: 'LEAD' },
+    })
+    const ma = await prisma.mentorAssignment.create({
+      data: {
+        projectId: project.id,
+        mentorId: mentor.id,
+        method: 'MANUAL',
+        workspaceEnabled: true,
+      },
+    })
+    for (let i = 0; i < 5; i++) {
+      await prisma.mentorMessage.create({
+        data: {
+          projectId: project.id,
+          senderId: mentor.id,
+          message: `mentor msg ${i}`,
+          workspaceId: ma.id,
+          isRead: i < 2,
+        },
+      })
+    }
+
+    const caller = createCaller(applicantRouter, {
+      id: applicant.id,
+      email: applicant.email,
+      role: 'APPLICANT',
+    })
+    const result = await caller.getMentorConversationPreview({ projectId: project.id })
+
+    expect(result.messages.length).toBe(3)
+    expect(result.unreadCount).toBe(3)
+    expect(result.mentor?.id).toBe(mentor.id)
+  })
+})