music: index artist discographies (album → release type) for the app

Each Albums/<Artist>/_discography.md (author-maintained source of truth, never
modified) is compiled into a per-artist discography.json in the cache = album
folder → normalized release type (Studio/Live/Compilation/Single/EP/…), so the
player can split an artist's album list into sections.

- indexer.ts: parse the md table, normalize the Type (EP?→EP, Compilation (VA)→
  Compilation, …), write discography.json. The artist folder's `v` now includes
  _discography.md so regenerating it re-syncs just that small JSON (isolated from
  the albums' meta/cover). Manifest gains `disco: true` on such entries. Also
  fixed the skip check to require all expected outputs to exist, so artist/
  cover-only folders no longer rebuild every run. New `discographies` counter.
- sidecar: GET /discography?path=<artist rel> (ETag/304), documented in the
  contract header.
- MUSIC_API.md: §2.4 + manifest disco flag + resync algorithm updated.

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

src/servers/sidecar/music/index.ts

@@ -10,6 +10,7 @@ import {
   albumVersion,
   metaFilePath,
   coverFilePath,
+  discographyFilePath,
   onIndexProgress,
   buildReport,
 } from './indexer';
@@ -30,9 +31,14 @@ const DATA_PATH = process.env.DATA_PATH ?? join(process.cwd(), 'data');
 //
 //   GET  /stream?path=<home-relative>      audio with Range→206 (Content-Range/Length/Accept-Ranges)
 //                                          + `X-Audio-Duration` (seconds, ffprobe). 400/404/416.
-//   GET  /manifest                         { version, generatedAt, albums: { "<rel>": { v, cover, tracks } } }
+//   GET  /manifest                         { version, generatedAt, albums: { "<rel>": { v, cover, tracks, disco? } } }
 //   GET  /meta?path=<rel>                  album meta.json (IndexMeta). ETag: <v>; If-None-Match → 304.
 //   GET  /cover?path=<rel>                 compressed cover.jpg.       ETag: <v>; If-None-Match → 304.
+//   GET  /discography?path=<artist rel>    artist discography.json (only where manifest entry has
+//                                          disco:true) = { artist, albums: { "<[year] album folder>":
+//                                          "<Type>" } }, Type ∈ Studio/Live/Compilation/Single/EP/…
+//                                          ETag: <v>; If-None-Match → 304. Source: each artist folder's
+//                                          _discography.md (normalized; the md itself is never modified).
 //   POST /reindex                          start an async build; returns IndexStatus (running: true).
 //   GET  /reindex/status                   IndexStatus snapshot.
 //   GET  /reindex/stream                   SSE. Triggers a build if idle (`?trigger=0` = watch-only).
@@ -135,21 +141,21 @@ const server = Bun.serve({
     // ── Sync surface ──
     if (url.pathname === '/manifest') return json(await getManifest());
 
-    if (url.pathname === '/meta' || url.pathname === '/cover') {
+    if (url.pathname === '/meta' || url.pathname === '/cover' || url.pathname === '/discography') {
       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 spec = {
+        '/meta': { file: metaFilePath(rel), type: 'application/json' },
+        '/cover': { file: coverFilePath(rel), type: 'image/jpeg' },
+        '/discography': { file: discographyFilePath(rel), type: 'application/json' },
+      }[url.pathname]!;
+      if (!spec.file) return new Response('Invalid path', { status: 400 });
+      if (!(await Bun.file(spec.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(Bun.file(spec.file), {
+        headers: { 'Content-Type': spec.type, ...(v ? { ETag: v } : {}) },
       });
     }