feat: heartbeat detection and sensor metrics

- Heartbeat service: check_sensor_status (online/stale/offline)
- GET /sensors/heartbeat: status summary for all sensors
- POST /sensors/heartbeat: refresh heartbeat status
- GET /sensors/<hostname>/metrics: activity counts, recent events
- CLI command: flask check-heartbeats
- Added 7 new tests (34 total)

TASKS.md

@@ -11,8 +11,8 @@
 - [x] `POST /api/v1/sensors/<id>/calibrate` — trigger calibration
 
 ### P2 - Normal
-- [ ] Sensor heartbeat timeout detection
-- [ ] Sensor metrics history endpoint
+- [x] Sensor heartbeat timeout detection
+- [x] Sensor metrics history endpoint
 
 ### P3 - Low
 - [ ] Add pagination to all list endpoints