- Add ActivitySample SQLAlchemy model for database persistence
- Add ActivityMonitor singleton with scoring, reset, lifecycle methods
- Add JUPYTERHUB_ACTIVITYMON_INACTIVE_AFTER env var (default 60 min)
- Update defaults: SAMPLE_INTERVAL=600s, RETENTION_DAYS=7
- Fix score calculation to use measured samples only (not theoretical max)
- Add 3-state status: green (active), yellow (inactive), red (offline)
- Add recently_active field in API response
- Add Reset button with confirmation dialog
- Fix green color (explicit #28a745 instead of text-success)
- Add ThreadPoolExecutor for non-blocking Docker stats
- Remove old background sampler code (on-demand sampling now)
- Bump version to 3.7.0