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/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);