feat(cache): CacheFiller background metadata sweeper

Periodic worker that walks cached_artists for missing album lists
and cached_albums for missing track lists, then fills them via
/api/artists/:id + /api/albums/:id. Newly-discovered album covers
are pre-warmed into flutter_cache_manager's disk cache too.

Solves the "tap an artist → empty album area → pop in" experience:
artistAlbumsProvider was drift-first but the cache only got
populated when the user navigated TO an artist. SyncController's
/api/library/sync delta doesn't carry per-artist album lists (those
are query-time derived). Now the filler pre-populates them in the
background so the drift hit is real on first tap.

Pacing (intentionally conservative):
* 10-second initial delay so SyncController has time to land its
  first sync — the WHERE NOT EXISTS query has nothing to do until
  cached_artists is populated.
* 5-minute interval thereafter. Once steady state is reached the
  sweep is a cheap drift query + early exit.
* 200ms throttle between per-entity REST requests — never competes
  with active playback.
* 200 entities per sweep cap so a fresh install with thousands of
  artists doesn't tie up the network for one continuous run. Next
  sweep picks up where this one left off (NOT EXISTS naturally
  skips already-filled rows).

Wall time estimate for a 1000-artist library: ~3-4 minutes spread
over multiple sweeps. Single round-trip per artist (new
getArtistDetail API method returns artist + albums in one shot).

Activated from app.dart's postFrameCallback alongside the existing
SyncController / Prefetcher / MetadataPrefetcher / LiveEvents
hooks. Disposed via ref.onDispose when the provider scope tears
down (effectively process death in practice).

flutter_client/lib/api/endpoints/library.dart

@@ -54,6 +54,24 @@ class LibraryApi {
     return ArtistRef.fromJson(r.data ?? const {});
   }
 
+  /// GET /api/artists/{id} — full response with the ArtistRef AND the
+  /// embedded album list, both parsed. Single round-trip variant used
+  /// by CacheFiller and other callers that want to populate both
+  /// cached_artists and cached_albums in one shot. The existing
+  /// getArtist / getArtistAlbums keep working for callers that only
+  /// need one half — they hit the same URL but the per-id Riverpod
+  /// caching layer dedupes.
+  Future<({ArtistRef artist, List<AlbumRef> albums})> getArtistDetail(
+      String id) async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/artists/$id');
+    final body = r.data ?? const <String, dynamic>{};
+    final artist = ArtistRef.fromJson(body);
+    final albums = ((body['albums'] as List?) ?? const [])
+        .map((e) => AlbumRef.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+    return (artist: artist, albums: albums);
+  }
+
   /// Pulls the "albums" array out of the same ArtistDetail body. Callers
   /// that need both the artist and its albums should issue two provider
   /// reads (artistProvider + artistAlbumsProvider) — both hit the same