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)

src/esp32_web/api/sensors.py

@@ -1,10 +1,12 @@
 """Sensor endpoints."""
 import json
 import socket
+from datetime import datetime, timedelta, UTC
 from flask import request, current_app
 from . import bp
-from ..models import Sensor
+from ..models import Sensor, Event, Sighting, Alert
 from ..extensions import db
+from ..services.heartbeat import get_heartbeat_summary, update_all_heartbeats
 
 
 @bp.route('/sensors')
@@ -213,3 +215,73 @@ def trigger_calibrate(hostname):
         return {'error': f'Socket error: {e}'}, 500
 
     return {'status': 'calibration_started', 'seconds': seconds}
+
+
+@bp.route('/sensors/heartbeat')
+def get_heartbeat_status():
+    """Get heartbeat status for all sensors."""
+    return get_heartbeat_summary()
+
+
+@bp.route('/sensors/heartbeat', methods=['POST'])
+def refresh_heartbeats():
+    """Update heartbeat status for all sensors."""
+    counts = update_all_heartbeats()
+    return {
+        'status': 'updated',
+        'online': counts['online'],
+        'stale': counts['stale'],
+        'offline': counts['offline']
+    }
+
+
+@bp.route('/sensors/<hostname>/metrics')
+def get_sensor_metrics(hostname):
+    """Get sensor activity metrics and recent events."""
+    sensor = db.session.scalar(db.select(Sensor).where(Sensor.hostname == hostname))
+    if not sensor:
+        return {'error': 'Sensor not found'}, 404
+
+    # Time range (default: last 24 hours)
+    hours = request.args.get('hours', 24, type=int)
+    if hours < 1 or hours > 168:  # max 1 week
+        hours = 24
+    since = datetime.now(UTC) - timedelta(hours=hours)
+
+    # Count activity
+    sightings_count = db.session.scalar(
+        db.select(db.func.count(Sighting.id))
+        .where(Sighting.sensor_id == sensor.id)
+        .where(Sighting.timestamp >= since)
+    ) or 0
+
+    alerts_count = db.session.scalar(
+        db.select(db.func.count(Alert.id))
+        .where(Alert.sensor_id == sensor.id)
+        .where(Alert.timestamp >= since)
+    ) or 0
+
+    events_count = db.session.scalar(
+        db.select(db.func.count(Event.id))
+        .where(Event.sensor_id == sensor.id)
+        .where(Event.timestamp >= since)
+    ) or 0
+
+    # Recent events (last 20)
+    recent_events = db.session.scalars(
+        db.select(Event)
+        .where(Event.sensor_id == sensor.id)
+        .order_by(Event.timestamp.desc())
+        .limit(20)
+    ).all()
+
+    return {
+        'hostname': sensor.hostname,
+        'hours': hours,
+        'activity': {
+            'sightings': sightings_count,
+            'alerts': alerts_count,
+            'events': events_count,
+        },
+        'recent_events': [e.to_dict() for e in recent_events]
+    }