feat: refactor MQTT from global config to multi-instance entity model

MQTT broker connections are now managed as entities (like HA sources)
instead of a single global config. Each MQTTSource gets its own
runtime with auto-reconnect, ref-counted via MQTTManager.

Backend:
- MQTTSource dataclass + MQTTSourceStore (SQLite)
- MQTTRuntime (per-broker connection, refactored from MQTTService)
- MQTTManager (ref-counted pool, same pattern as HAManager)
- CRUD API at /api/v1/mqtt/sources + test + status endpoints
- MQTTRule gains mqtt_source_id field for source selection
- Automation engine acquires/releases MQTT runtimes automatically
- Legacy MQTTService kept for backward compat during transition

Frontend:
- MQTT source cards in Streams > Integrations tab
- Create/edit modal with test button
- Dashboard integration cards with health-dot indicators
- Removed MQTT tab from settings modal

server/src/wled_controller/api/__init__.py

@@ -26,6 +26,7 @@ from .routes.weather_sources import router as weather_sources_router
 from .routes.update import router as update_router
 from .routes.assets import router as assets_router
 from .routes.home_assistant import router as home_assistant_router
+from .routes.mqtt import router as mqtt_router
 from .routes.game_integration import router as game_integration_router
 
 router = APIRouter()
@@ -53,6 +54,7 @@ router.include_router(weather_sources_router)
 router.include_router(update_router)
 router.include_router(assets_router)
 router.include_router(home_assistant_router)
+router.include_router(mqtt_router)
 router.include_router(game_integration_router)
 
 __all__ = ["router"]