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/services/ai-shortlist.ts

@@ -14,7 +14,7 @@ import { getOpenAI, getConfiguredModel, buildCompletionParams } from '@/lib/open
 import { logAIUsage, extractTokenUsage } from '@/server/utils/ai-usage'
 import { classifyAIError, logAIError } from './ai-errors'
 import { extractMultipleFileContents } from './file-content-extractor'
-import type { PrismaClient } from '@prisma/client'
+import type { PrismaClient, CompetitionCategory } from '@prisma/client'
 
 // ─── Types ──────────────────────────────────────────────────────────────────
 
@@ -95,14 +95,14 @@ async function generateCategoryShortlist(
     rubric?: string
     aiParseFiles: boolean
   },
-  prisma: PrismaClient | any,
+  prisma: PrismaClient,
 ): Promise<{ recommendations: ShortlistRecommendation[]; tokensUsed: number; errors: string[] }> {
   const { roundId, category, topN, rubric, aiParseFiles } = params
 
   // Load projects with evaluations for this category
   const projects = await prisma.project.findMany({
     where: {
-      competitionCategory: category,
+      competitionCategory: category as CompetitionCategory,
       assignments: { some: { roundId } },
     },
     include: {
@@ -320,7 +320,7 @@ export async function generateShortlist(
     rubric?: string
     aiParseFiles?: boolean
   },
-  prisma: PrismaClient | any,
+  prisma: PrismaClient,
 ): Promise<ShortlistResult> {
   const {
     roundId,