M8-R01: add configurable MQTT client identity

tests/test_api_config.py

@@ -454,6 +454,7 @@ def test_get_config_includes_mqtt_section(client: TestClient) -> None:
     assert "MQTT_USERNAME" in env_names
     assert "MQTT_PASSWORD" in env_names
     assert "MQTT_TLS_ENABLED" in env_names
+    assert "MQTT_CLIENT_ID" in env_names
 
 
 def test_get_config_includes_ha_discovery_section(client: TestClient) -> None:
@@ -578,6 +579,18 @@ def test_put_config_invalid_mqtt_port_returns_422_and_does_not_write(
     assert rows.get("MQTT_BROKER_PORT") != "not-a-number"
 
 
+def test_put_config_invalid_mqtt_client_id_returns_422(client: TestClient) -> None:
+    _login(client)
+
+    response = client.put(
+        "/api/config",
+        json={"updates": _full_config_payload({"MQTT_CLIENT_ID": "invalid id"})},
+        headers={"X-CSRF-Token": "token"},
+    )
+
+    assert response.status_code == 422
+
+
 # ---------------------------------------------------------------------------
 # M5-polish2 Area B: bool fields have input_type="checkbox"
 # ---------------------------------------------------------------------------
@@ -722,6 +735,32 @@ def test_put_config_mqtt_reconnect_uses_db_merged_settings(
     )
 
 
+def test_put_config_mqtt_client_id_reconnects_and_trims_value(
+    client: TestClient, test_database_urls
+) -> None:
+    _login(client)
+    mock_mgr = MagicMock()
+
+    with patch("app.api.routes.api.config.mqtt_manager", mock_mgr):
+        response = client.put(
+            "/api/config",
+            json={"updates": _full_config_payload({"MQTT_CLIENT_ID": " home-automation-dev "})},
+            headers={"X-CSRF-Token": "token"},
+        )
+
+    assert response.status_code == 200
+    reconnect_settings = mock_mgr.reconnect.call_args.args[0]
+    assert reconnect_settings.mqtt_client_id == "home-automation-dev"
+    conn = sqlite3.connect(test_database_urls["app_path"])
+    try:
+        stored_value = conn.execute(
+            "SELECT value FROM app_config WHERE key = 'MQTT_CLIENT_ID'"
+        ).fetchone()
+    finally:
+        conn.close()
+    assert stored_value == ("home-automation-dev",)
+
+
 # ---------------------------------------------------------------------------
 # M6-T02: DSMR + Tibber CONFIG_FIELDS
 # ---------------------------------------------------------------------------