fix(02): Alpine libmpv/libsndfile symlinks, upload endpoint, browser file picker

- Dockerfile: add libmpv.so + libsndfile.so symlinks (Alpine/musl has no ldconfig)
- engine.py: monkey-patch ctypes.util.find_library as fallback for Alpine
- audio.py: add /upload (multipart) and /files endpoints
- pyproject.toml: add python-multipart
- frontend: replace path input with upload button + file selector dropdown

lightsync/api/audio.py

@@ -1,13 +1,15 @@
 """Audio loading and status endpoints."""
 import asyncio
+import shutil
 from pathlib import Path
-from fastapi import APIRouter, HTTPException, Request
+from fastapi import APIRouter, HTTPException, Request, UploadFile, File
 from pydantic import BaseModel
 from lightsync.audio.waveform import extract_peaks
 
 router = APIRouter()
 
 SUPPORTED_EXTENSIONS = {".mp3", ".wav", ".flac", ".ogg"}
+SHOWS_DIR = Path("/app/shows")
 
 
 class LoadRequest(BaseModel):
@@ -48,6 +50,38 @@ async def get_audio_state(request: Request):
     return engine.get_state()
 
 
+@router.get("/files")
+async def list_audio_files():
+    """List audio files available in the shows directory."""
+    SHOWS_DIR.mkdir(parents=True, exist_ok=True)
+    files = sorted(
+        p.name for p in SHOWS_DIR.iterdir()
+        if p.is_file() and p.suffix.lower() in SUPPORTED_EXTENSIONS
+    )
+    return {"files": files, "directory": str(SHOWS_DIR)}
+
+
+@router.post("/upload")
+async def upload_audio(request: Request, file: UploadFile = File(...)):
+    """Upload an audio file to the shows directory and load it."""
+    engine = request.app.state.engine
+    if engine is None:
+        raise HTTPException(status_code=503, detail="Audio engine not ready")
+
+    suffix = Path(file.filename).suffix.lower()
+    if suffix not in SUPPORTED_EXTENSIONS:
+        raise HTTPException(status_code=400, detail=f"Unsupported format: {suffix}. Supported: {', '.join(sorted(SUPPORTED_EXTENSIONS))}")
+
+    SHOWS_DIR.mkdir(parents=True, exist_ok=True)
+    dest = SHOWS_DIR / file.filename
+
+    with dest.open("wb") as f:
+        shutil.copyfileobj(file.file, f)
+
+    await asyncio.to_thread(engine.load, str(dest))
+    return {"status": "loaded", "path": str(dest), "filename": file.filename}
+
+
 @router.get("/waveform")
 async def get_waveform(request: Request, peaks: int = 1000):
     """Get waveform peak data for the currently loaded audio file (AUD-05).