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>

prisma/migrations/20260308000000_drop_dead_models_and_stale_columns/migration.sql

@@ -0,0 +1,35 @@
+-- DropForeignKey
+ALTER TABLE "AdvancementRule" DROP CONSTRAINT "AdvancementRule_roundId_fkey";
+
+-- DropForeignKey
+ALTER TABLE "AssignmentException" DROP CONSTRAINT "AssignmentException_approvedById_fkey";
+
+-- DropForeignKey
+ALTER TABLE "AssignmentException" DROP CONSTRAINT "AssignmentException_assignmentId_fkey";
+
+-- AlterTable
+ALTER TABLE "ConflictOfInterest" DROP COLUMN "roundId";
+
+-- AlterTable
+ALTER TABLE "Evaluation" DROP COLUMN "version";
+
+-- AlterTable
+ALTER TABLE "Project" DROP COLUMN "roundId";
+
+-- DropTable
+DROP TABLE "AdvancementRule";
+
+-- DropTable
+DROP TABLE "AssignmentException";
+
+-- DropTable
+DROP TABLE "NotificationPolicy";
+
+-- DropTable
+DROP TABLE "OverrideAction";
+
+-- DropEnum
+DROP TYPE "AdvancementRuleType";
+
+-- DropEnum
+DROP TYPE "OverrideReasonCode";