Implement security hardening, notification pipeline, and CI test suite

Security hardening:

- Added IP/user rate limiting with Redis-backed counters and fail-open behavior.

- Added message anti-spam controls (per-chat rate + duplicate cooldown).

- Implemented refresh token rotation with JTI tracking and revoke support.

Notification pipeline:

- Added Celery app and async notification tasks for mention/offline delivery.

- Added Redis-based presence tracking and integrated it into realtime connect/disconnect.

- Added notification dispatch from message flow and notifications listing endpoint.

Quality gates and CI:

- Added pytest async integration tests for auth and chat/message lifecycle.

- Added pytest config, test fixtures, and GitHub Actions CI workflow.

- Fixed bcrypt/passlib compatibility by pinning bcrypt version.

- Documented worker and quality-gate commands in README.

README.md

@@ -10,3 +10,16 @@ Backend foundation for a Telegram-like real-time messaging platform.
 3. Configure environment from `.env.example`.
 4. Start API:
    uvicorn app.main:app --reload --port 8000
+
+## Celery Worker
+
+Run worker for async notification jobs:
+
+celery -A app.celery_app:celery_app worker --loglevel=info
+
+## Quality Gates
+
+- Compile check:
+  python -m compileall app main.py
+- Tests:
+  pytest -q