feat: rewrite busylight as FastAPI web application with SQLite

Replace the standalone Python script + config.cfg with a full web
application featuring:
- FastAPI backend with REST API
- SQLite database (config, state, activity log separated)
- Bootstrap 5 web interface for configuration
- Pages: Dashboard, Users, Devices, Assignments, Color Rules, Settings, Log
- APScheduler background polling for Teams presence + AIDA status
- systemd service file for Linux deployment
- Migration script to import existing config.cfg data

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

requirements.txt

@@ -0,0 +1,8 @@
+fastapi>=0.104.0
+uvicorn[standard]>=0.24.0
+jinja2>=3.1.0
+python-multipart>=0.0.6
+requests>=2.31.0
+azure-identity>=1.15.0
+msgraph-core>=0.2.2
+apscheduler>=3.10.0