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/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>