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>

backend/requirements-dev.txt

@@ -4,6 +4,7 @@
 # Testing — pytest-asyncio 0.24+ requires pytest>=8.2
 pytest==8.4.2
 pytest-asyncio==0.24.0
+pytest-xdist==3.6.1
 httpx>=0.27.0
 pytest-cov==5.0.0