Skip integration tests in CI — no Postgres service available

The migrations-idempotency.integration.test.js requires a reachable
Postgres instance. The CI Docker container can't resolve the DATABASE_URL
hostname. Skip files matching 'integration' in the test-backend job.

.gitlab-ci.yml

@@ -100,7 +100,7 @@ test-backend:
     policy: pull
   script:
     - test -d node_modules || npm ci
-    - ./node_modules/.bin/jest --ci --forceExit backend/__tests__/
+    - ./node_modules/.bin/jest --ci --forceExit --testPathIgnorePatterns='integration' backend/__tests__/
   timeout: 5 minutes
   needs:
     - install-backend