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/api/subsonic.contract.test.ts

@@ -11,7 +11,8 @@
  * mocking and are not in this PR.
  */
 import { beforeEach, describe, expect, it } from 'vitest';
-import { buildCoverArtUrl, buildDownloadUrl, buildStreamUrl, coverArtCacheKey, parseSubsonicEntityStarRating } from './subsonic';
+import { buildCoverArtUrl, buildDownloadUrl, buildStreamUrl, coverArtCacheKey } from './subsonic';
+import { parseSubsonicEntityStarRating } from './subsonicRatings';
 import { getClient, libraryFilterParams } from './subsonicClient';
 import { useAuthStore } from '@/store/authStore';
 import { resetAuthStore } from '@/test/helpers/storeReset';