refactor: tech debt batch 2 — drop dead models, stale columns, schema cleanup

Schema:
- Drop 4 dead models: OverrideAction, NotificationPolicy, AssignmentException, AdvancementRule
- Drop 2 dead enums: OverrideReasonCode, AdvancementRuleType
- Drop 3 stale columns: Project.roundId, ConflictOfInterest.roundId, Evaluation.version
- Remove 3 back-relation fields from User, Assignment, Round

Code:
- Fix 6 COI queries in assignment.ts + 1 in juror-reassignment.ts
  (roundId filter → assignment.roundId after column drop)
- Remove orphaned Project.roundId write in project.ts createProject
- Remove advancementRules include from round.ts getById
- Remove AdvancementRule from RoundWithRelations type
- Clean up seed.ts (remove advancement rule seeding)
- Clean up tests/helpers.ts (remove dead model cleanup)
- Add TODO comments on user delete mutations (FK violation risk)

Migration: 20260308000000_drop_dead_models_and_stale_columns

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

src/server/routers/assignment.ts

@@ -119,7 +119,7 @@ async function runAIAssignmentJob(jobId: string, roundId: string, userId: string
     // Query COI records for this round to exclude conflicted juror-project pairs
     const coiRecords = await prisma.conflictOfInterest.findMany({
       where: {
-        roundId,
+        assignment: { roundId },
         hasConflict: true,
       },
       select: { userId: true, projectId: true },
@@ -1665,7 +1665,7 @@ export const assignmentRouter = router({
       for (const a of existingAssignments) currentLoads.set(a.userId, (currentLoads.get(a.userId) ?? 0) + 1)
 
       const coiRecords = await ctx.prisma.conflictOfInterest.findMany({
-        where: { roundId: input.roundId, hasConflict: true, userId: { in: candidateIds } },
+        where: { assignment: { roundId: input.roundId }, hasConflict: true, userId: { in: candidateIds } },
         select: { userId: true, projectId: true },
       })
       const coiPairs = new Set(coiRecords.map((c) => `${c.userId}:${c.projectId}`))
@@ -1911,7 +1911,7 @@ export const assignmentRouter = router({
 
       const coiRecords = await ctx.prisma.conflictOfInterest.findMany({
         where: {
-          roundId: input.roundId,
+          assignment: { roundId: input.roundId },
           hasConflict: true,
           userId: { in: candidateIds },
         },
@@ -2026,7 +2026,7 @@ export const assignmentRouter = router({
 
       const coiRecords = await ctx.prisma.conflictOfInterest.findMany({
         where: {
-          roundId: input.roundId,
+          assignment: { roundId: input.roundId },
           hasConflict: true,
           userId: { in: destinationIds },
         },
@@ -2367,7 +2367,7 @@ export const assignmentRouter = router({
 
       const coiRecords = await ctx.prisma.conflictOfInterest.findMany({
         where: {
-          roundId: input.roundId,
+          assignment: { roundId: input.roundId },
           hasConflict: true,
           userId: { in: candidateIds },
         },