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)

tests/test_api/test_sensors.py

@@ -1,7 +1,7 @@
 """Sensor API tests."""
 from unittest.mock import patch, MagicMock
 from esp32_web.extensions import db
-from esp32_web.models import Sensor
+from esp32_web.models import Sensor, Event
 
 
 def test_list_sensors_empty(client):
@@ -215,3 +215,123 @@ def test_trigger_calibrate_default_seconds(client, app):
                                json={})
         assert response.status_code == 200
         assert response.json['seconds'] == 10
+
+
+# Heartbeat Tests
+
+def test_heartbeat_status_empty(client):
+    """Test heartbeat status with no sensors."""
+    response = client.get('/api/v1/sensors/heartbeat')
+    assert response.status_code == 200
+    assert response.json['total'] == 0
+    assert response.json['sensors'] == []
+
+
+def test_heartbeat_status_with_sensors(client, app):
+    """Test heartbeat status with sensors."""
+    from datetime import datetime, UTC, timedelta
+
+    with app.app_context():
+        # Online sensor (just now)
+        s1 = Sensor(hostname='sensor-online', ip='192.168.1.1',
+                    last_seen=datetime.now(UTC))
+        # Stale sensor (3 minutes ago)
+        s2 = Sensor(hostname='sensor-stale', ip='192.168.1.2',
+                    last_seen=datetime.now(UTC) - timedelta(minutes=3))
+        # Offline sensor (10 minutes ago)
+        s3 = Sensor(hostname='sensor-offline', ip='192.168.1.3',
+                    last_seen=datetime.now(UTC) - timedelta(minutes=10))
+        db.session.add_all([s1, s2, s3])
+        db.session.commit()
+
+    response = client.get('/api/v1/sensors/heartbeat')
+    assert response.status_code == 200
+    assert response.json['total'] == 3
+    assert response.json['online'] == 1
+    assert response.json['stale'] == 1
+    assert response.json['offline'] == 1
+
+
+def test_refresh_heartbeats(client, app):
+    """Test refreshing heartbeat status."""
+    from datetime import datetime, UTC, timedelta
+
+    with app.app_context():
+        # Offline sensor but status still says 'online'
+        sensor = Sensor(hostname='test-sensor', ip='192.168.1.1',
+                        last_seen=datetime.now(UTC) - timedelta(minutes=10),
+                        status='online')
+        db.session.add(sensor)
+        db.session.commit()
+
+    response = client.post('/api/v1/sensors/heartbeat')
+    assert response.status_code == 200
+    assert response.json['status'] == 'updated'
+    assert response.json['offline'] == 1
+
+    # Verify status was updated
+    with app.app_context():
+        sensor = db.session.scalar(db.select(Sensor).where(Sensor.hostname == 'test-sensor'))
+        assert sensor.status == 'offline'
+
+
+# Metrics Tests
+
+def test_metrics_not_found(client):
+    """Test metrics for non-existent sensor."""
+    response = client.get('/api/v1/sensors/nonexistent/metrics')
+    assert response.status_code == 404
+
+
+def test_metrics_empty(client, app):
+    """Test metrics for sensor with no activity."""
+    with app.app_context():
+        sensor = Sensor(hostname='test-sensor', ip='192.168.1.100')
+        db.session.add(sensor)
+        db.session.commit()
+
+    response = client.get('/api/v1/sensors/test-sensor/metrics')
+    assert response.status_code == 200
+    assert response.json['hostname'] == 'test-sensor'
+    assert response.json['hours'] == 24
+    assert response.json['activity']['sightings'] == 0
+    assert response.json['activity']['alerts'] == 0
+    assert response.json['activity']['events'] == 0
+    assert response.json['recent_events'] == []
+
+
+def test_metrics_with_events(client, app):
+    """Test metrics with sensor events."""
+    from datetime import datetime, UTC
+
+    with app.app_context():
+        sensor = Sensor(hostname='test-sensor', ip='192.168.1.100')
+        db.session.add(sensor)
+        db.session.flush()
+
+        # Add some events
+        event1 = Event(sensor_id=sensor.id, event_type='presence',
+                       payload_json='{"state": "detected"}',
+                       timestamp=datetime.now(UTC))
+        event2 = Event(sensor_id=sensor.id, event_type='calibration',
+                       payload_json='{"nsub": 52}',
+                       timestamp=datetime.now(UTC))
+        db.session.add_all([event1, event2])
+        db.session.commit()
+
+    response = client.get('/api/v1/sensors/test-sensor/metrics')
+    assert response.status_code == 200
+    assert response.json['activity']['events'] == 2
+    assert len(response.json['recent_events']) == 2
+
+
+def test_metrics_custom_hours(client, app):
+    """Test metrics with custom time range."""
+    with app.app_context():
+        sensor = Sensor(hostname='test-sensor', ip='192.168.1.100')
+        db.session.add(sensor)
+        db.session.commit()
+
+    response = client.get('/api/v1/sensors/test-sensor/metrics?hours=48')
+    assert response.status_code == 200
+    assert response.json['hours'] == 48