fix: make EvaluationForm constraint drop idempotent (IF EXISTS)

The migration failed on deploy because the constraint
EvaluationForm_roundId_version_key did not exist in the target DB.
Using DROP CONSTRAINT IF EXISTS makes this safe for databases where
the constraint was already removed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

prisma/migrations/20260331000000_add_evaluation_form_category/migration.sql

@@ -1,8 +1,9 @@
 -- AlterTable: add nullable category column to EvaluationForm
 ALTER TABLE "EvaluationForm" ADD COLUMN "category" "CompetitionCategory";
 
--- Drop old unique constraint
-ALTER TABLE "EvaluationForm" DROP CONSTRAINT "EvaluationForm_roundId_version_key";
+-- Drop old unique constraint (IF EXISTS — may not exist on fresh databases where
+-- the constraint was never created or was already removed by an earlier migration)
+ALTER TABLE "EvaluationForm" DROP CONSTRAINT IF EXISTS "EvaluationForm_roundId_version_key";
 
 -- Add new unique constraint including category
 ALTER TABLE "EvaluationForm" ADD CONSTRAINT "EvaluationForm_roundId_version_category_key" UNIQUE ("roundId", "version", "category");