feat: pagination totals, request logging, data retention

Add shared paginate() helper with total count to all list endpoints.
Add request logging middleware (method, path, status, duration, IP).
Add data retention service with configurable thresholds and CLI command.

tests/test_api/test_sensors.py

@@ -8,7 +8,10 @@ def test_list_sensors_empty(client):
     """Test listing sensors when empty."""
     response = client.get('/api/v1/sensors')
     assert response.status_code == 200
-    assert response.json == {'sensors': []}
+    assert response.json['sensors'] == []
+    assert response.json['total'] == 0
+    assert response.json['limit'] == 100
+    assert response.json['offset'] == 0
 
 
 def test_get_sensor_not_found(client):