feat(02-02): wire WebSocket broadcast loop and command dispatch

- Add broadcast_loop sending position ticks at 10Hz to all clients
- Replace echo stub with play/pause/seek/load command dispatch to MPVEngine
- Start broadcast_loop as asyncio task in lifespan, cancel on shutdown
- Use get_engine callable to avoid circular import at module level

lightsync/main.py

@@ -1,3 +1,4 @@
+import asyncio
 import os
 from contextlib import asynccontextmanager
 from pathlib import Path
@@ -27,8 +28,15 @@ async def lifespan(app: FastAPI):
     engine = MPVEngine(ao=ao)
     engine.start()
     app.state.engine = engine
+    from lightsync.api.ws import broadcast_loop
+    task = asyncio.create_task(broadcast_loop(lambda: engine))
     yield
     # Shutdown
+    task.cancel()
+    try:
+        await task
+    except asyncio.CancelledError:
+        pass
     engine.stop()
     await registry.save()