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

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
     "python-mpv>=1.0.5",
     "soundfile>=0.12.0",
     "numpy>=1.26.0",
+    "python-multipart>=0.0.9",
 ]
 
 [project.optional-dependencies]