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>

tests/unit/mentor-round-stats.test.ts

@@ -92,6 +92,7 @@ describe('mentor.getRoundStats', () => {
     })
     await prisma.mentorFile.create({
       data: {
+        projectId: projReqAssigned.id,
         mentorAssignmentId: a1.id,
         uploadedByUserId: mentor.id,
         fileName: 'plan.pdf',