docs(spotify): note intentional Spotify omission from unified /search/all (stage 1)

Spotify tracks are metadata-only until the librespot audio backend lands, so
they are surfaced only from the dedicated Spotify tab, not the default
all-sources search. Conscious decision from the whole-branch review (#112).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/api/music.ts

@@ -115,6 +115,11 @@ export function createMusicRouter(
         return;
       }
       const parsedLimit = parseInt(limit as string) || 20;
+      // Spotify is intentionally EXCLUDED from unified /search/all in Stage 1:
+      // its tracks are metadata-only (not yet playable) until the librespot audio
+      // backend lands (Stage 2/3), so surfacing them in the default all-sources
+      // view would only yield results that get skipped. Spotify search remains
+      // available from its own tab via /search?platform=spotify.
       const [neteaseResult, qqResult, bilibiliResult, localResult, kugouResult] = await Promise.allSettled([
         neteaseProvider.search(q as string, parsedLimit),
         qqProvider.search(q as string, parsedLimit),