perf(ci): pytest-xdist with per-worker DBs — 22m → ~4m

Backend suite is the slow gate (1076 passed locally in 22m27s on
fix/ci-workflow-config). Adding pytest-xdist with per-worker DB
isolation drops it to ~4m20s on the 8-core homelab runner. Verified
locally: `pytest -n auto --no-cov` finished in 4m28s real time
(15m19s user — confirms ~5× parallelism).

How it works:
- conftest.py reads `PYTEST_XDIST_WORKER` (set per worker by xdist —
  'gw0', 'gw1', …). When set, derives a per-worker DB URL like
  `…/resolutionflow_test_gw0`. The base DB stays for serial / master
  runs.
- `_ensure_worker_db_exists` runs synchronously at conftest import,
  connects to the postgres maintenance DB, and `CREATE DATABASE`s the
  worker-suffixed DB if it doesn't exist. Idempotent across runs.
- The "test" safety guard still applies — every worker DB name
  contains "test" so the assertion holds.
- The per-test `DROP SCHEMA public CASCADE` now operates on the
  worker's isolated DB, no cross-worker race.

CI workflow: backend job switches to `pytest -n auto`. Coverage still
collected (pytest-cov has built-in xdist support).

Adds `pytest-xdist==3.6.1` to requirements-dev.txt.

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

.gitea/workflows/ci.yml

@@ -69,11 +69,15 @@ jobs:
         run: cd backend && python scripts/check_tenant_filters.py
 
       - name: Run tests with coverage
+        # `-n auto` parallelizes across all runner cores via pytest-xdist.
+        # conftest.py creates a per-worker DB (resolutionflow_test_gw0,
+        # resolutionflow_test_gw1, …) so the per-test DROP SCHEMA doesn't
+        # race across workers. Master/serial runs keep the base DB.
         # term-missing dropped — the custom "Display coverage summary" step
         # below parses coverage.json and prints the same info more concisely.
         # --maxfail=10 short-circuits on structural breakage so we don't burn
         # 25 minutes when a fixture explodes.
-        run: cd backend && python -m pytest --override-ini="addopts=" --maxfail=10 --cov=app --cov-report=json:coverage.json --cov-fail-under=50
+        run: cd backend && python -m pytest --override-ini="addopts=" -n auto --maxfail=10 --cov=app --cov-report=json:coverage.json --cov-fail-under=50
 
       - name: Display coverage summary
         if: always()