refactor: tech debt batch 3 — type safety + assignment router split

#5 — Replaced 55x PrismaClient | any with proper Prisma types across 8 files
- Service files: PrismaClient | any → PrismaClient, tx: any → Prisma.TransactionClient
- Fixed 4 real bugs uncovered by typing:
  - mentor-workspace.ts: wrong FK fields (mentorAssignmentId → workspaceId, role → senderRole)
  - ai-shortlist.ts: untyped string passed to CompetitionCategory enum filter
  - result-lock.ts: unknown passed where Prisma.InputJsonValue required

#9 — Split assignment.ts (2,775 lines) into 6 focused files:
  - shared.ts (93 lines) — MOVABLE_EVAL_STATUSES, buildBatchNotifications, getCandidateJurors
  - assignment-crud.ts (473 lines) — 8 core CRUD procedures
  - assignment-suggestions.ts (880 lines) — AI suggestions + job runner
  - assignment-notifications.ts (138 lines) — 2 notification procedures
  - assignment-redistribution.ts (1,162 lines) — 8 reassign/transfer procedures
  - index.ts (15 lines) — barrel export with router merge, zero frontend changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/server/routers/mentor.ts

@@ -1343,7 +1343,7 @@ export const mentorRouter = router({
     .mutation(async ({ ctx, input }) => {
       return workspaceSendMessage(
         {
-          mentorAssignmentId: input.mentorAssignmentId,
+          workspaceId: input.mentorAssignmentId,
           senderId: ctx.user.id,
           message: input.message,
           role: input.role,
@@ -1389,7 +1389,7 @@ export const mentorRouter = router({
     .mutation(async ({ ctx, input }) => {
       return workspaceUploadFile(
         {
-          mentorAssignmentId: input.mentorAssignmentId,
+          workspaceId: input.mentorAssignmentId,
           uploadedByUserId: ctx.user.id,
           fileName: input.fileName,
           mimeType: input.mimeType,