refactor(api): F.49 — extract library + artists + ratings (#614)

Three domain-eng modules peel ~316 LOC of fetch/mapping out of
`api/subsonic.ts`:

- `subsonicLibrary.ts` — browse + random + per-song fetch
  (`getMusicDirectory`, `getMusicIndexes`, `getMusicFolders`,
  `getRandomAlbums`, `getAlbumList`, `getRandomSongs`,
  `getRandomSongsFiltered`, `getSong`, `getAlbum`,
  `filterSongsToActiveLibrary`, `similarSongsRequestCount`, plus
  the private `albumIdsInActiveLibraryScope` cache).
- `subsonicArtists.ts` — artist endpoints (`getArtists`, `getArtist`,
  `getArtistInfo`, `getTopSongs`, `getSimilarSongs2`,
  `getSimilarSongs`). Uses Library's `filterSongsToActiveLibrary` and
  `similarSongsRequestCount` for the per-library scoping fallback.
- `subsonicRatings.ts` — `parseSubsonicEntityStarRating` parser plus
  `prefetchArtistUserRatings` and `prefetchAlbumUserRatings` workers
  with the shared 7-min cache. Calls back into Library/Artists for
  the per-id fetch.

51 external call sites migrated to direct imports. No re-export
shims in `subsonic.ts`. Statistics endpoints still in `subsonic.ts`
keep their `RATING_CACHE_TTL` constant locally (same 7-min window).

subsonic.ts: 1078 → 762 LOC (−316).

src/test/mocks/subsonic.ts

@@ -7,7 +7,7 @@
  *
  *   // In the test:
  *   vi.mock('@/api/subsonic');
- *   import { getAlbum, buildStreamUrl } from '@/api/subsonic';
+ *   import { buildStreamUrl } from '@/api/subsonic';
  *   import { sampleAlbumWithSongs, mockStreamUrl } from '@/test/mocks/subsonic';
  *
  *   beforeEach(() => {
@@ -18,6 +18,7 @@
  * Realistic shape matters more than perfect coverage — these fixtures
  * mirror what Navidrome actually returns for common queries.
  */
+import { getAlbum } from '@/api/subsonicLibrary';
 import type { SubsonicSong, SubsonicAlbum, SubsonicPlaylist } from '@/api/subsonicTypes';
 import { makeSubsonicSong } from '@/test/helpers/factories';