refactor(api): F.50 — extract 7 small domain modules (#615)

Seven domain-eng splits peel ~200 LOC of read endpoints out of
`api/subsonic.ts`:

- `subsonicStreamUrl.ts` — `buildStreamUrl`, `coverArtCacheKey`,
  `buildCoverArtUrl`, `buildDownloadUrl` (token-signed URL builders
  for the four /rest endpoints we hand to the browser).
- `subsonicStarRating.ts` — `getStarred`, `star`, `unstar`,
  `setRating`, `probeEntityRatingSupport`. `setRating` still triggers
  the lazy `navidromeBrowse` cache invalidation; the same-folder
  lazy import path is preserved.
- `subsonicSearch.ts` — `search`, `searchSongsPaged`.
- `subsonicScrobble.ts` — `scrobbleSong`, `reportNowPlaying`,
  `getNowPlaying`.
- `subsonicAlbumInfo.ts` — `getAlbumInfo2`.
- `subsonicLyrics.ts` — `getLyricsBySongId`.
- `subsonicGenres.ts` — `getGenres`, `getAlbumsByGenre`.

63 external call sites migrated to direct imports. Four `vi.mock`
targets in the store-level tests pointed at `../api/subsonic` and
were updated to the new module paths.

Pure code-move. subsonic.ts: 762 → 561 LOC (−201).

src/test/mocks/subsonic.ts

@@ -7,8 +7,7 @@
  *
  *   // In the test:
  *   vi.mock('@/api/subsonic');
- *   import { buildStreamUrl } from '@/api/subsonic';
- *   import { sampleAlbumWithSongs, mockStreamUrl } from '@/test/mocks/subsonic';
+ *    *   import { sampleAlbumWithSongs, mockStreamUrl } from '@/test/mocks/subsonic';
  *
  *   beforeEach(() => {
  *     vi.mocked(getAlbum).mockResolvedValue(sampleAlbumWithSongs);
@@ -18,6 +17,7 @@
  * Realistic shape matters more than perfect coverage — these fixtures
  * mirror what Navidrome actually returns for common queries.
  */
+import { buildStreamUrl } from '@/api/subsonicStreamUrl';
 import { getAlbum } from '@/api/subsonicLibrary';
 import type { SubsonicSong, SubsonicAlbum, SubsonicPlaylist } from '@/api/subsonicTypes';
 import { makeSubsonicSong } from '@/test/helpers/factories';