Add pytest-xdist and run the backend test suite across multiple workers
(`-n 4` in CI). Each worker gets its own database so the autouse
`clear_database` fixture can't wipe rows another worker is mid-test with:
- Rootdir `backend/conftest.py` sets a per-worker `DB_NAME`
(`romm_test_gw0`, ...) before any app module is imported, so each
worker's engine binds to its own database.
- `tests/conftest.py` creates the per-worker database on demand (mariadb/
mysql and postgresql paths) just before migrations run.
- The test user's grant is widened to `*.*` (setup.sql + CI) so it can
`CREATE DATABASE` for the workers.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
