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

Dockerfile

@@ -7,7 +7,9 @@ RUN apk add --no-cache \
     mpv-libs \
     ffmpeg \
     ffmpeg-libs \
-    libsndfile
+    libsndfile && \
+    ln -sf /usr/lib/libmpv.so.2 /usr/lib/libmpv.so && \
+    ln -sf /usr/lib/libsndfile.so.1 /usr/lib/libsndfile.so
 
 COPY pyproject.toml .
 RUN pip install --no-cache-dir .