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/project.ts

@@ -638,10 +638,6 @@ export const projectRouter = router({
         })
 
         if (input.roundId) {
-          await tx.project.update({
-            where: { id: created.id },
-            data: { roundId: input.roundId },
-          })
           await tx.projectRoundState.create({
             data: {
               projectId: created.id,