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/user.ts

@@ -206,7 +206,10 @@ export const userRouter = router({
         })
       }
 
-      // Delete user
+      // TODO: This delete will fail with a FK violation for any user with activity
+      // (COI declarations, mentor assignments, messages, evaluations, etc.).
+      // A proper purge flow with pre-deletion cleanup or soft-delete is needed
+      // before hard-delete can work reliably for active users.
       await ctx.prisma.user.delete({
         where: { id: ctx.user.id },
       })
@@ -657,6 +660,10 @@ export const userRouter = router({
         select: { email: true },
       })
 
+      // TODO: This delete will fail with a FK violation for any user with activity
+      // (COI declarations, mentor assignments, messages, evaluations, etc.).
+      // A proper purge flow with pre-deletion cleanup or soft-delete is needed
+      // before hard-delete can work reliably for active users.
       const user = await ctx.prisma.user.delete({
         where: { id: input.id },
       })