M8-R01: add configurable MQTT client identity

tests/test_mqtt_subscribe.py

@@ -251,6 +251,7 @@ def test_replace_source_uses_isolated_client_and_source_credentials() -> None:
             password="one-secret",
             tls_enabled=True,
             subscriptions={"one/topic": lambda payload: received.append(("one", payload))},
+            base_client_id="home-automation-dev",
         )
         manager.replace_source(
             2,
@@ -284,6 +285,25 @@ def test_replace_source_uses_isolated_client_and_source_credentials() -> None:
     assert received == [("two", b"two"), ("changed", b"changed")]
 
 
+def test_source_client_id_is_scoped_by_deployment_and_source() -> None:
+    manager = MqttManager()
+    client = MagicMock()
+
+    with patch("app.integrations.mqtt.mqtt.Client", return_value=client) as mock_cls:
+        assert manager.replace_source(
+            42,
+            host="broker.test",
+            port=1883,
+            username="",
+            password="",
+            tls_enabled=False,
+            subscriptions={"topic": lambda _payload: None},
+            base_client_id="home-automation-dev",
+        )
+
+    assert mock_cls.call_args.kwargs["client_id"] == "home-automation-dev-dsmr-source-42"
+
+
 def test_replaced_source_client_callbacks_cannot_reach_new_generation() -> None:
     """A retained old paho client cannot subscribe, mutate state, or dispatch new handlers."""
     manager = MqttManager()