refactor(lib): move uploadArtistImage to subsonicArtists (M5 misplacement)

uploadArtistImage lived in lib/api/subsonicPlaylists but is artist-domain and
consumed only by features/artist/runArtistDetailActions. Relocated to
lib/api/subsonicArtists (its rightful API module); consumer repointed. Also fixes
a stale doc comment in features/randomMix/index.ts (mixRatingFilter now lives in
features/playback). No behavior change; tests pass unmodified.

src/features/artist/utils/runArtistDetailActions.ts

@@ -1,6 +1,6 @@
 import type React from 'react';
 import type { TFunction } from 'i18next';
-import { uploadArtistImage } from '@/lib/api/subsonicPlaylists';
+import { uploadArtistImage } from '@/lib/api/subsonicArtists';
 import { setRating, star, unstar } from '@/lib/api/subsonicStarRating';
 import type { SubsonicArtist } from '@/lib/api/subsonicTypes';
 import { useAuthStore } from '@/store/authStore';

src/features/randomMix/index.ts

@@ -5,9 +5,9 @@
  * random-mix queue-build helpers. The pages are lazy-loaded by the router via
  * their deep paths, so they are not re-exported here.
  *
- * Stays OUT (shared infra, consumed by the playback core too, not owned):
- * `utils/mix/mixRatingFilter` (a generic rating-window filter used by the
- * infinite-queue builder + home/album/CLI → a `lib/` candidate).
+ * Stays OUT (owned by the playback feature, consumed cross-feature here):
+ * `features/playback/utils/mixRatingFilter` (rating-window filter driven by the
+ * infinite-queue builder; reads playback's userRatingOverrides).
  */
 export { useLuckyMixAvailable, isLuckyMixAvailable } from './hooks/useLuckyMixAvailable';
 export { useLuckyMixStore } from './store/luckyMixStore';