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/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: