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).

lightsync/audio/engine.py

@@ -1,10 +1,22 @@
 """MPV-based audio engine — headless playback with 10Hz position polling."""
+import ctypes.util as _ctypes_util
 import sys
 import threading
 import time
 from typing import Any
 
-import mpv
+# Alpine/musl: ctypes.util.find_library can't discover libmpv without ldconfig or objdump.
+# Patch it to return the known path so python-mpv can load the library.
+_orig_find_library = _ctypes_util.find_library
+def _patched_find_library(name: str) -> str | None:
+    if name == "mpv":
+        return "/usr/lib/libmpv.so.2"
+    return _orig_find_library(name)
+_ctypes_util.find_library = _patched_find_library
+
+import mpv  # noqa: E402
+
+_ctypes_util.find_library = _orig_find_library  # restore after mpv loaded
 
 
 def apply_timer_fix() -> None:

lightsync/frontend/app.js

@@ -224,10 +224,55 @@ if (seekBar) {
     });
 }
 
-// Load audio button
+// Populate file selector
+async function refreshFileList(selectValue) {
+    const sel = document.getElementById('audio-select');
+    if (!sel) return;
+    try {
+        const res = await fetch('/api/audio/files');
+        if (!res.ok) return;
+        const { files } = await res.json();
+        const prev = selectValue ?? sel.value;
+        sel.innerHTML = '<option value="">— select file —</option>';
+        files.forEach(name => {
+            const opt = document.createElement('option');
+            opt.value = `/app/shows/${name}`;
+            opt.textContent = name;
+            if (opt.value === prev) opt.selected = true;
+            sel.appendChild(opt);
+        });
+    } catch (e) {
+        console.error('[audio] file list error:', e);
+    }
+}
+refreshFileList();
+
+// Upload audio file
+document.getElementById('audio-upload')?.addEventListener('change', async (e) => {
+    const file = e.target.files?.[0];
+    if (!file) return;
+    const form = new FormData();
+    form.append('file', file);
+    try {
+        const res = await fetch('/api/audio/upload', { method: 'POST', body: form });
+        if (!res.ok) {
+            const err = await res.json();
+            console.error('[audio] upload failed:', err.detail);
+            return;
+        }
+        const { path } = await res.json();
+        await refreshFileList(path);
+        setTimeout(loadWaveform, 500);
+    } catch (err) {
+        console.error('[audio] upload error:', err);
+    }
+    e.target.value = '';
+});
+
+// Load selected audio file
 document.getElementById('btn-load')?.addEventListener('click', async () => {
-    const pathInput = document.getElementById('audio-path');
-    const path = pathInput?.value.trim();
+    const sel = document.getElementById('audio-select');
+    const path = sel?.value;
     if (!path) return;
     try {
         const res = await fetch('/api/audio/load', {
@@ -240,7 +285,6 @@ document.getElementById('btn-load')?.addEventListener('click', async () => {
             console.error('[audio] load failed:', err.detail);
             return;
         }
-        // Wait briefly for mpv to initialize, then fetch waveform
         setTimeout(loadWaveform, 500);
     } catch (err) {
         console.error('[audio] load error:', err);

lightsync/frontend/index.html

@@ -72,8 +72,14 @@
                 <input type="range" id="seek-bar" min="0" max="100" value="0" step="0.1" class="seek-slider">
             </div>
             <div class="transport-file">
-                <input type="text" id="audio-path" placeholder="Audio file path on server..." class="audio-path-input">
-                <button id="btn-load" class="transport-btn" title="Load audio">LOAD</button>
+                <label class="transport-btn" title="Upload MP3/WAV/FLAC/OGG from your computer" style="cursor:pointer;">
+                    + UPLOAD
+                    <input type="file" id="audio-upload" accept=".mp3,.wav,.flac,.ogg" style="display:none;">
+                </label>
+                <select id="audio-select" class="audio-path-input">
+                    <option value="">— no files yet, upload first —</option>
+                </select>
+                <button id="btn-load" class="transport-btn" title="Load selected file">LOAD</button>
             </div>
         </footer>