feat(02-02): add REST audio load endpoint with file validation

- Create lightsync/api/audio.py with POST /load and GET /state endpoints
- Validate file existence and extension (.mp3/.wav/.flac/.ogg)
- Use asyncio.to_thread for engine.load to avoid blocking event loop
- Return 404/400/503 on error conditions
- Register audio router at /api/audio prefix in main.py
- AUD-02 (YouTube) noted as Phase 6 deferral

lightsync/main.py

@@ -45,10 +45,11 @@ def create_app() -> FastAPI:
     app = FastAPI(title="LightSync", lifespan=lifespan, redirect_slashes=False)
 
     # API routes (registered before static mount — see Pattern 7)
-    from lightsync.api import shows, devices, ws
+    from lightsync.api import shows, devices, ws, audio
     app.include_router(shows.router, prefix="/api/shows")
     app.include_router(devices.router, prefix="/api/devices")
     app.include_router(ws.router)
+    app.include_router(audio.router, prefix="/api/audio")
 
     # Static file serving — no-cache headers to prevent stale JS/CSS
     frontend_dir = Path(__file__).parent / "frontend"