music: server-side library indexer + sync surface

The officer-music sidecar now builds a cache tree mirroring the library (server
counterpart of the app's music-index.ts), and exposes an rsync-clean diff surface.

Indexer (indexer.ts): walks HOME_DIR/Music; per album computes a version `v` =
hash of the source signature (track name+size+mtime, cover size+mtime); ffprobe
→ meta.json (phone IndexMeta schema: file/title/artist/albumArtist/album/track/
year/durationSec); ffmpeg compresses the cover to <=600px q5 cover.jpg. Writes
DATA_PATH/music/cache/<rel>/. Incremental (skip albums whose `v` is unchanged),
prunes cache dirs for albums removed from the library, maintains manifest.json.

Endpoints (sidecar, auto-proxied by /api/music/*):
  POST /reindex          async build; GET /reindex/status polls progress
  GET  /manifest         { version, albums: { "<rel>": { v, cover, tracks } } }
  GET  /meta?path=<rel>  album meta.json   (ETag: v, 304 on If-None-Match)
  GET  /cover?path=<rel> compressed cover  (ETag: v, 304 on If-None-Match)

Phone resync: GET /manifest, diff `v` against last-stored → fetch only changed
albums' meta+cover; drop rels missing from the manifest. No re-download of
unchanged albums.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/sidecar/music/index.ts

@@ -1,6 +1,14 @@
 import type { SidecarCommand, SidecarEvent } from '../protocol';
 import { createSidecarConnector } from '../connect';
 import { streamAudioFile } from './stream-audio';
+import {
+  buildMusicIndex,
+  getIndexStatus,
+  getManifest,
+  albumVersion,
+  metaFilePath,
+  coverFilePath,
+} from './indexer';
 
 // The music sidecar (officer-music). Same philosophy as the other officer-* sidecars: a singleton
 // process that registers with the API server. It OWNS an audio-streaming HTTP server (all path
@@ -28,12 +36,47 @@ const server = Bun.serve({
   hostname: '127.0.0.1',
   async fetch(req) {
     const url = new URL(req.url);
+    const json = (data: unknown, init?: ResponseInit) =>
+      new Response(JSON.stringify(data), { ...init, headers: { 'Content-Type': 'application/json', ...init?.headers } });
+
     if (url.pathname === '/health') return new Response('ok');
+
+    // ── Streaming ──
     if (url.pathname === '/stream') {
       const path = url.searchParams.get('path');
       if (!path) return new Response('path is required', { status: 400 });
       return streamAudioFile(path, req.headers.get('range'));
     }
+
+    // ── Index build ──
+    if (url.pathname === '/reindex') {
+      if (req.method !== 'POST') return new Response('Method not allowed', { status: 405 });
+      void buildMusicIndex(); // fire-and-forget; sets running=true synchronously before the first await
+      return json(getIndexStatus());
+    }
+    if (url.pathname === '/reindex/status') return json(getIndexStatus());
+
+    // ── Sync surface ──
+    if (url.pathname === '/manifest') return json(await getManifest());
+
+    if (url.pathname === '/meta' || url.pathname === '/cover') {
+      const rel = url.searchParams.get('path');
+      if (rel === null) return new Response('path is required', { status: 400 });
+      const isMeta = url.pathname === '/meta';
+      const file = isMeta ? metaFilePath(rel) : coverFilePath(rel);
+      if (!file) return new Response('Invalid path', { status: 400 });
+      if (!(await Bun.file(file).exists())) return new Response('Not found', { status: 404 });
+
+      const v = await albumVersion(rel);
+      if (v && req.headers.get('if-none-match') === v) return new Response(null, { status: 304 });
+      return new Response(Bun.file(file), {
+        headers: {
+          'Content-Type': isMeta ? 'application/json' : 'image/jpeg',
+          ...(v ? { ETag: v } : {}),
+        },
+      });
+    }
+
     return new Response('Not found', { status: 404 });
   },
 });