Update Python version requirements and fix compatibility issues

- Bump minimum Python version from 3.11 to 3.13 across all apps
- Add Python 3.11-3.13 test matrix to CLI workflow
- Document Python 3.11+ requirement in .env.example
- Fix Starlette Broadcast removal with in-process fallback implementation
- Add _InProcessBroadcast class for tests when Starlette Broadcast is unavailable
- Refactor API key validators to read live settings instead of cached values
- Update database models with explicit

systemd/aitbc-coordinator-api.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=AITBC Coordinator API Service
+Description=AITBC Coordinator API Service (Python 3.11+)
 After=network.target
 Wants=network.target
 
@@ -10,6 +10,8 @@ Group=oib
 WorkingDirectory=/opt/coordinator-api
 Environment=PATH=/opt/coordinator-api/.venv/bin
 Environment=PYTHONPATH=/opt/coordinator-api/src
+# Python version validation
+ExecStartPre=/bin/bash -c "python3.11 --version || (echo 'Python 3.11+ required' && exit 1)"
 ExecStart=/opt/coordinator-api/.venv/bin/python -m uvicorn app.main:app --host 0.0.0.0 --port 8000
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=always