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.

.github/workflows/ci.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+
+      - name: Compile check
+        run: |
+          python -m compileall app main.py
+
+      - name: Run tests
+        run: |
+          pytest -q