refactor(schema-cascade): rename Project.mentorAssignment → mentorAssignments

Schema dropped @unique on MentorAssignment.projectId in PR8 Task 1 →
back-relation becomes a list. Mechanical rename of Prisma queries and
consumer accessors. Legacy single-mentor callers use [0] with a TODO for
PR8 Task 8 to surface the full list. mentor-workspace.ts is left as Task 5.

- routers (mentor, project, applicant, finalist, round) and smart-assignment
  service: include/where/select keys renamed; `mentorAssignment: null` →
  `mentorAssignments: { none: {} }`; `{ isNot: null }` → `{ some: {} }`.
- UI consumers (mentor + applicant pages): `project.mentorAssignment` →
  `project.mentorAssignments[0]` with TODO markers.
- Tests: `findUnique({ projectId })` → `findFirst({ projectId })` since the
  composite key now requires both projectId+mentorId. MentorFile.create gains
  the new required projectId.
- Workspace endpoints in mentor.ts now guard null mentorAssignmentId until
  Task 5 re-scopes them to project.
- finalist.unconfirm now cascades to ALL active mentor assignments.

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

src/app/(mentor)/mentor/projects/[id]/page.tsx

@@ -94,14 +94,18 @@ function ProjectDetailContent({ projectId }: { projectId: string }) {
     },
   })
 
+  // TODO(PR8 Task 9): show co-mentors. For now we pick the first assignment
+  // to keep tracking + chat working unchanged.
+  const primaryAssignment = project?.mentorAssignments?.[0] ?? null
+
   // Track view when project loads
   const trackView = trpc.mentor.trackView.useMutation()
   useEffect(() => {
-    if (project?.mentorAssignment?.id) {
-      trackView.mutate({ mentorAssignmentId: project.mentorAssignment.id })
+    if (primaryAssignment?.id) {
+      trackView.mutate({ mentorAssignmentId: primaryAssignment.id })
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [project?.mentorAssignment?.id])
+  }, [primaryAssignment?.id])
 
   if (isLoading) {
     return <ProjectDetailSkeleton />
@@ -135,7 +139,7 @@ function ProjectDetailContent({ projectId }: { projectId: string }) {
 
   const teamLead = project.teamMembers?.find((m) => m.role === 'LEAD')
   const otherMembers = project.teamMembers?.filter((m) => m.role !== 'LEAD') || []
-  const mentorAssignment = project.mentorAssignment
+  const mentorAssignment = primaryAssignment
   const mentorAssignmentId = mentorAssignment?.id
   const programId = project.program?.id
   const viewerIsAssignedMentor =
@@ -477,7 +481,7 @@ function ProjectDetailContent({ projectId }: { projectId: string }) {
         <CardContent>
           <MentorChat
             messages={mentorMessages || []}
-            currentUserId={project.mentorAssignment?.mentor?.id || ''}
+            currentUserId={primaryAssignment?.mentor?.id || ''}
             onSendMessage={async (message) => {
               await sendMessage.mutateAsync({ projectId, message })
             }}