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/(applicant)/applicant/mentor/page.tsx

@@ -72,7 +72,10 @@ export default function ApplicantMentorPage() {
     )
   }
 
-  const mentor = dashboardData?.project?.mentorAssignment?.mentor
+  // TODO(PR8 Task 7): show ALL assigned mentors. For now we display only the
+  // first one until the multi-mentor applicant UI ships.
+  const primaryAssignment = dashboardData?.project?.mentorAssignments?.[0] ?? null
+  const mentor = primaryAssignment?.mentor
 
   return (
     <div className="space-y-6">
@@ -136,9 +139,9 @@ export default function ApplicantMentorPage() {
       )}
 
       {/* Files */}
-      {dashboardData?.project?.mentorAssignment?.id && (
+      {primaryAssignment?.id && (
         <WorkspaceFilesPanel
-          mentorAssignmentId={dashboardData.project.mentorAssignment.id}
+          mentorAssignmentId={primaryAssignment.id}
           asApplicant
         />
       )}