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/compose.prod.yml

@@ -58,7 +58,11 @@ services:
       BANGUI_FAIL2BAN_SOCKET: "/var/run/fail2ban/fail2ban.sock"
       BANGUI_FAIL2BAN_CONFIG_DIR: "/config/fail2ban"
       BANGUI_LOG_LEVEL: "info"
-      BANGUI_WORKERS: "1"  # APScheduler requires single worker — do not change
+      # ⚠️  BANGUI_WORKERS MUST be 1 — see session_cache.py docstring for details
+      # BanGUI uses a process-local session cache. Multiple workers in a single process
+      # would cause users to be randomly logged out as sessions wouldn't be shared.
+      # For HA, run multiple BanGUI instances (each with --workers 1) via orchestration.
+      BANGUI_WORKERS: "1"
       BANGUI_SESSION_SECRET: "${BANGUI_SESSION_SECRET:?Set BANGUI_SESSION_SECRET}"
       BANGUI_TIMEZONE: "${BANGUI_TIMEZONE:-UTC}"
     volumes: