feat(mqtt): multi-broker MQTT + Zigbee2MQTT light target

- New Z2MLightOutputTarget storage, processor, editor and routes for
  Zigbee2MQTT light entities (shares the HA-Light editor UI via the new
  light-target-editor module)
- Replace global MQTTService/MQTTConfig with per-source MQTTManager +
  MQTTRuntime; thread mqtt_source_id through Z2M targets, DIY MQTT
  devices, and the automation engine
- Migrate legacy single-broker YAML/env config to a "Default Broker"
  MQTTSource on startup (core/mqtt/legacy_migration.py) and drop the
  obsolete core/mqtt/mqtt_service.py
- Refresh /api/v1/system integration status to surface every MQTT source
- Extract shared light-target editor and refactor OutputTargetStore +
  output_targets routes around typed factories / auto-registry
- Modal CSS polish, locale strings, and storage/bindable test coverage

server/tests/test_config.py

@@ -37,10 +37,6 @@ class TestDefaultConfig:
         reload(paths_mod)
         assert paths_mod.default_data_dir() == Path(str(tmp_path / "custom"))
 
-    def test_default_mqtt_disabled(self):
-        config = Config()
-        assert config.mqtt.enabled is False
-
     def test_default_demo_off(self):
         config = Config()
         assert config.demo is False