chore(tests): gate RLS tests behind RUN_RLS_TESTS flag

Continues the test-isolation work from dab740d. RLS migration tests run
against a policy-installed database and fail in the default create_all
suite, so they need to be opt-in:

- pytest.ini: register `rls` marker.
- conftest.py: auto-deselect test_rls_isolation.py unless
  RUN_RLS_TESTS=1. Drops the deprecated session-scoped event_loop
  fixture (not needed since pytest-asyncio 0.23+).
- test_rls_isolation.py: tag module with `rls` marker. Replace
  hardcoded `patherly_test` DB reference with parsed DATABASE_TEST_URL
  (matches conftest.py default `resolutionflow_test`). Updated docstring
  command to show RUN_RLS_TESTS=1.
- requirements-dev.txt: bump pytest-asyncio 0.23.0 → 0.24.0 (loop-scope
  marker behavior required by the RLS module fixture).

Run the RLS suite with:
  RUN_RLS_TESTS=1 DB_APP_ROLE_PASSWORD=... pytest tests/test_rls_isolation.py

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/requirements-dev.txt

@@ -3,7 +3,7 @@
 
 # Testing
 pytest==7.4.3
-pytest-asyncio==0.23.0
+pytest-asyncio==0.24.0
 httpx>=0.27.0
 pytest-cov==4.1.0