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/index.ts

@@ -1,3 +1,5 @@
+import { mkdirSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
 import type { SidecarCommand, SidecarEvent } from '../protocol';
 import { createSidecarConnector } from '../connect';
 import { streamAudioFile } from './stream-audio';
@@ -8,8 +10,12 @@ import {
   albumVersion,
   metaFilePath,
   coverFilePath,
+  onIndexProgress,
+  buildReport,
 } from './indexer';
 
+const DATA_PATH = process.env.DATA_PATH ?? join(process.cwd(), 'data');
+
 // 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
 // resolution + streaming + ffprobe duration happens here); the platform API is just a thin proxy that
@@ -56,6 +62,54 @@ const server = Bun.serve({
     }
     if (url.pathname === '/reindex/status') return json(getIndexStatus());
 
+    // SSE progress stream (for the app + the CLI). Triggers a build if idle (unless ?trigger=0), then
+    // streams `progress` events until the build finishes, ending with a `done` event carrying the report.
+    if (url.pathname === '/reindex/stream') {
+      const trigger = url.searchParams.get('trigger') !== '0';
+      if (trigger && !getIndexStatus().running) void buildMusicIndex();
+
+      const encoder = new TextEncoder();
+      const stream = new ReadableStream({
+        start(controller) {
+          let closed = false;
+          let unsub = () => {};
+          const send = (event: string, data: unknown) => {
+            if (closed) return;
+            try {
+              controller.enqueue(encoder.encode(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`));
+            } catch {
+              /* stream closed */
+            }
+          };
+          const finish = (s: ReturnType<typeof getIndexStatus>) => {
+            send('done', buildReport(s));
+            unsub();
+            closed = true;
+            try {
+              controller.close();
+            } catch {
+              /* already closed */
+            }
+          };
+
+          send('progress', getIndexStatus());
+          const cur = getIndexStatus();
+          if (!cur.running) {
+            finish(cur); // nothing running → emit the last report and close
+            return;
+          }
+          unsub = onIndexProgress((s) => {
+            send('progress', s);
+            if (!s.running && s.finishedAt) finish(s);
+          });
+        },
+      });
+
+      return new Response(stream, {
+        headers: { 'Content-Type': 'text/event-stream', 'Cache-Control': 'no-cache', Connection: 'keep-alive' },
+      });
+    }
+
     // ── Sync surface ──
     if (url.pathname === '/manifest') return json(await getManifest());
 
@@ -83,6 +137,14 @@ const server = Bun.serve({
 
 console.log(`[music] audio server listening on http://127.0.0.1:${port}`);
 
+// Write the port to a well-known file so local tooling (scripts/reindex-music.ts) can find the server.
+try {
+  mkdirSync(join(DATA_PATH, 'music'), { recursive: true });
+  writeFileSync(join(DATA_PATH, 'music', '.server'), String(port));
+} catch {
+  /* best-effort */
+}
+
 // ── Command handlers ──
 
 type ReplyFn = (msg: SidecarEvent) => void;