test: real Redis integration suite and CI service

[mamahoos]

Summary

• Add `tests/integration/` against a real Redis server (`REDIS_URL`)
• New workflow `redis-integration.yml` with `redis:7-alpine` service
• Unit CI excludes `integration` marker; fakeredis stays for fast units
• Fix overwrite race: write storage marker before aborting previous wait

Test plan

• `pytest -m "not integration"` → 42 passed
• `REDIS_URL=... pytest tests/integration -m integration` → 7 passed
• GitHub Actions `Redis Integration` job green