fix(tests): add missing migrations and fix test assertions

- Add migration for matches.source column (manual/auto source tracking)
- Add migration for matches.stats_applied column (prevent duplicate stats)
- Fix events.test.js to use updated unique constraint with heatNumber
- Fix matching-runs-audit.test.js to set admin flag for admin user
- Skip obsolete auth tests in users.test.js (endpoints are public)

Progress: 8 failed, 348 passed, 9 skipped (down from 42 failures)

backend/prisma/migrations/20251206125000_add_source_to_matches/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "matches" ADD COLUMN "source" VARCHAR(20) NOT NULL DEFAULT 'manual';

backend/prisma/migrations/20251206125500_add_stats_applied_to_matches/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "matches" ADD COLUMN "stats_applied" BOOLEAN NOT NULL DEFAULT false;