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

.env.example

@@ -1,5 +1,10 @@
 # AITBC Environment Configuration
 # Copy this file to .env and fill in your values
+#
+# Requirements:
+# - Python 3.11 or later
+# - SQLite or PostgreSQL database
+# - Bitcoin node (for wallet integration)
 
 # Coordinator API
 APP_ENV=dev