feat(02-01): implement MPVEngine with position polling and lifespan integration

- Create lightsync/audio/__init__.py package marker
- Create lightsync/audio/engine.py with MPVEngine class
  - Headless mpv wrapper (vo=null, ao from MPV_AO env var default null)
  - 10Hz polling loop in daemon thread (time.sleep(0.1), not observe_property)
  - Thread-safe get_state() returning position, paused, duration, loaded, file
  - load/play/pause/seek/stop methods (AUD-01, AUD-03)
  - apply_timer_fix() with sys.platform == win32 guard (INF-03)
- Update main.py lifespan to create/start/stop MPVEngine
  - MPV_AO env var controls audio output (default null for server mode)
  - engine exposed via app.state.engine for API routes

lightsync/main.py

@@ -1,3 +1,4 @@
+import os
 from contextlib import asynccontextmanager
 from pathlib import Path
 from fastapi import FastAPI, Request
@@ -6,22 +7,29 @@ from fastapi.staticfiles import StaticFiles
 
 from lightsync.devices.registry import DeviceRegistry
 from lightsync.store.show_store import ShowStore
+from lightsync.audio.engine import MPVEngine
 
 # Module-level containers populated in lifespan
 registry: DeviceRegistry | None = None
 show_store: ShowStore | None = None
+engine: MPVEngine | None = None
 
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    global registry, show_store
+    global registry, show_store, engine
     # Startup
     registry = DeviceRegistry(Path("devices.json"))
     await registry.load()
     show_store = ShowStore(Path("shows"))
     show_store.ensure_dir()
+    ao = os.environ.get("MPV_AO", "null")
+    engine = MPVEngine(ao=ao)
+    engine.start()
+    app.state.engine = engine
     yield
     # Shutdown
+    engine.stop()
     await registry.save()