Fix: Enforce single-worker deployment for session cache cluster safety

Addresses: Backend session cache not cluster-safe (multi-worker issue)

Problem:
- Session cache is process-local (InMemorySessionCache)
- Multi-worker deployments (uvicorn --workers N) create separate processes
- Each process has its own independent session cache
- Sessions cached in Worker A are invisible to Workers B, C, D
- Users randomly logged out when requests land on different workers
- Also affects RuntimeState, rate limiter, and background jobs

Solution (Option A - Strict single-worker enforcement):
- Enhance startup validation with clearer error messages
- Update error messages to explain the problem and how to fix it
- Document single-worker requirement prominently in Docker configs
- Update module docstrings to clarify constraints

Changes:
1. app/startup.py:
   - Enhanced _check_single_worker_mode() error message with troubleshooting
   - Enhanced _stage_check_worker_mode_and_acquire_lock() error message
   - Removed unused import

2. app/utils/session_cache.py:
   - Updated module docstring to explain constraints more clearly
   - Added references to deployment documentation
   - Clarified multi-worker solution for future implementation

3. app/utils/runtime_state.py:
   - Updated module docstring with deployment constraint references
   - Aligned messaging with session_cache.py

4. Docker/Dockerfile.backend:
   - Added comprehensive comments about single-worker requirement
   - Explained impact in multi-worker deployments
   - Referenced deployment constraints documentation

5. Docker/docker-compose.yml, compose.prod.yml, compose.debug.yml:
   - Added documentation comments about BANGUI_WORKERS constraint
   - Explained why single-worker is required

6. backend/tests/test_startup_integration.py:
   - Fixed test unpacking to match function return signature (3 values, not 2)

This ensures multi-worker deployments fail loudly at startup with clear
guidance on what went wrong and how to fix it. The database-backed scheduler
lock provides defense-in-depth for container orchestration scenarios.

For future multi-worker support, implement:
- Redis or database-backed session cache
- Shared RuntimeState coordination
- Distributed APScheduler backend

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docker/Dockerfile.backend

@@ -67,4 +67,19 @@ USER bangui
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
     CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/api/health')" || exit 1
 
+# ⚠️  IMPORTANT: Single-Worker Requirement
+# BanGUI must always run as a single worker process:
+#   - Do NOT pass --workers or --worker-class to uvicorn
+#   - Do NOT use gunicorn with -w 4 or similar
+#   - Do NOT override BANGUI_WORKERS to > 1
+#
+# Why? The session cache is process-local. Multiple workers would cause:
+#   - Random user logouts (sessions not shared between workers)
+#   - Duplicate background jobs (each worker runs the scheduler)
+#   - SQLite lock contention and timeouts
+#
+# For high availability, use container orchestration (Kubernetes, Docker Swarm)
+# to run multiple instances, not multiple workers in a single process.
+#
+# See Docs/Architekture.md § Deployment Constraints for details.
 CMD ["uvicorn", "app.main:create_app", "--factory", "--host", "0.0.0.0", "--port", "8000"]