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/api/audio.py

@@ -0,0 +1,47 @@
+"""Audio loading and status endpoints."""
+import asyncio
+from pathlib import Path
+from fastapi import APIRouter, HTTPException, Request
+from pydantic import BaseModel
+
+router = APIRouter()
+
+SUPPORTED_EXTENSIONS = {".mp3", ".wav", ".flac", ".ogg"}
+
+
+class LoadRequest(BaseModel):
+    path: str
+
+
+class AudioState(BaseModel):
+    position: float
+    paused: bool
+    duration: float | None
+    loaded: bool
+    file: str | None
+
+
+@router.post("/load")
+async def load_audio(req: LoadRequest, request: Request):
+    """Load an audio file by server path (AUD-01). YouTube URL deferred to Phase 6 (AUD-02 stub)."""
+    engine = request.app.state.engine
+    if engine is None:
+        raise HTTPException(status_code=503, detail="Audio engine not ready")
+
+    p = Path(req.path)
+    if not p.exists():
+        raise HTTPException(status_code=404, detail=f"File not found: {req.path}")
+    if p.suffix.lower() not in SUPPORTED_EXTENSIONS:
+        raise HTTPException(status_code=400, detail=f"Unsupported format: {p.suffix}. Supported: {', '.join(sorted(SUPPORTED_EXTENSIONS))}")
+
+    await asyncio.to_thread(engine.load, req.path)
+    return {"status": "loaded", "path": req.path}
+
+
+@router.get("/state")
+async def get_audio_state(request: Request):
+    """Get current audio playback state."""
+    engine = request.app.state.engine
+    if engine is None:
+        raise HTTPException(status_code=503, detail="Audio engine not ready")
+    return engine.get_state()

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"