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>

.gitignore

@@ -0,0 +1,12 @@
+__pycache__/
+*.pyc
+*.pyo
+.env
+venv/
+busylight.db
+config.cfg
+config.dev.cfg
+*.egg-info/
+dist/
+build/
+.vscode/