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>

backend/tests/test_startup_integration.py

@@ -83,11 +83,12 @@ async def test_startup_shared_resources_complete_flow() -> None:
             mock_blocklist_import_register.return_value = None
 
             # Call startup_shared_resources
-            http_session, scheduler = await startup_shared_resources(app, settings)
+            http_session, scheduler, startup_db = await startup_shared_resources(app, settings)
 
             # Verify all stages completed successfully
             assert http_session is not None
             assert scheduler is not None
+            assert startup_db is not None
             assert scheduler.running
 
             # Verify resources were initialized
@@ -178,7 +179,7 @@ async def test_startup_shared_resources_scheduler_starts() -> None:
             mock_geo_cache.init_geoip = MagicMock()
             mock_geo_cache_class.return_value = mock_geo_cache
 
-            http_session, scheduler = await startup_shared_resources(app, settings)
+            http_session, scheduler, startup_db = await startup_shared_resources(app, settings)
 
             # Verify scheduler is running
             assert scheduler.running