music: SSE reindex progress stream + reindex-music CLI

Adds a live progress channel for the library index:
- indexer.ts: progress subscribers (onIndexProgress) + throttled emit during the
  walk, and buildReport() for a final summary.
- sidecar: GET /reindex/stream (SSE) — triggers a build if idle (?trigger=0 to
  watch only), streams `progress` events, ends with a `done` event carrying the
  report; auto-proxied at /api/music/reindex/stream for the app. Sidecar also
  writes DATA_PATH/music/.server (its port) for local tooling.
- scripts/reindex-music.ts: CLI that reads the port file, follows the SSE, prints
  live progress + a final report. Run: bun scripts/reindex-music.ts

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

src/servers/sidecar/music/indexer.ts

@@ -101,6 +101,57 @@ export function getIndexStatus(): IndexStatus {
   return { ...status };
 }
 
+export type IndexReport = {
+  albums: number; // albums with content (built + skipped)
+  built: number;
+  skipped: number;
+  foldersScanned: number;
+  tracksIndexed: number;
+  coversSaved: number;
+  elapsedSec: number;
+  error: string | null;
+};
+
+export function buildReport(s: IndexStatus = status): IndexReport {
+  const elapsed = s.startedAt && s.finishedAt ? (s.finishedAt - s.startedAt) / 1000 : 0;
+  return {
+    albums: s.albumsBuilt + s.albumsSkipped,
+    built: s.albumsBuilt,
+    skipped: s.albumsSkipped,
+    foldersScanned: s.foldersScanned,
+    tracksIndexed: s.tracksIndexed,
+    coversSaved: s.coversSaved,
+    elapsedSec: Math.round(elapsed * 10) / 10,
+    error: s.error,
+  };
+}
+
+// ── Progress subscribers (for SSE) ──
+
+type ProgressListener = (s: IndexStatus) => void;
+const listeners = new Set<ProgressListener>();
+let lastEmit = 0;
+
+export function onIndexProgress(cb: ProgressListener): () => void {
+  listeners.add(cb);
+  return () => listeners.delete(cb);
+}
+
+/** Notify subscribers of progress; throttled to ~200ms unless `force` (e.g. terminal 'done'). */
+function emitProgress(force = false): void {
+  const now = Date.now();
+  if (!force && now - lastEmit < 200) return;
+  lastEmit = now;
+  const snap = getIndexStatus();
+  for (const cb of listeners) {
+    try {
+      cb(snap);
+    } catch {
+      /* ignore listener errors */
+    }
+  }
+}
+
 // ── Helpers ──
 
 function isAudio(name: string): boolean {
@@ -226,6 +277,7 @@ export async function buildMusicIndex(): Promise<IndexStatus> {
     status.running = false;
     status.finishedAt = Date.now();
     status.currentPath = '';
+    emitProgress(true); // final push — signals 'done' to SSE subscribers
   }
   return getIndexStatus();
 }
@@ -233,6 +285,7 @@ export async function buildMusicIndex(): Promise<IndexStatus> {
 async function walk(dirAbs: string, prev: Manifest, next: Manifest): Promise<void> {
   status.currentPath = relative(MUSIC_ROOT, dirAbs) || '.';
   status.foldersScanned += 1;
+  emitProgress();
 
   let entries: import('node:fs').Dirent[];
   try {
@@ -276,6 +329,7 @@ async function walk(dirAbs: string, prev: Manifest, next: Manifest): Promise<voi
         const tracks = await mapPool(audio, TRACK_CONCURRENCY, async (name) => {
           const t = await ffprobeTrack(join(dirAbs, name), name);
           status.tracksIndexed += 1;
+          emitProgress();
           return t;
         });
         const meta: IndexMeta = { path: rel, cover: coverName ? 'cover.jpg' : undefined, tracks };