library: materialize artist art and provider images

Store the OpenSubsonic artistImageUrl and two materialized cover-id lists on each artist row, backfill them in the 35->36 migration, and refresh downloaded covers only for affected artists when downloads change. Icons and the artist hero now resolve provider art, server artist art, then the same materialized grid synchronously, with a definitive-failure fallback so a dead provider URL does not strand the letter placeholder.

Download artwork also caches the provider image for the track and album artists, falls back to the artist cover when that fetch fails, and keeps both keys across sweeps. Query timing for the materialization pass is down from roughly 3.4s to 1.9s on the 20k-song benchmark.

app/src/main/kotlin/com/subtracks/ui/search/SearchScreen.kt

@@ -46,6 +46,7 @@ import com.subtracks.data.model.Playlist
 import com.subtracks.data.model.Song
 import com.subtracks.data.source.StarType
 import com.subtracks.playback.PlaybackController
+import com.subtracks.ui.components.ArtistArt
 import com.subtracks.ui.components.ContextMenuHost
 import com.subtracks.ui.components.EmptyState
 import com.subtracks.ui.components.ItemActions
@@ -106,7 +107,7 @@ fun SearchRoute(
             if (next != 0) filterMask = next
         },
         coverArt = viewModel::coverArt,
-        artistCovers = viewModel::artistArtCovers,
+        artistArt = { viewModel.artistArt(it) },
         playingSongId = playback.item?.id,
         onBack = onBack,
         onAlbumClick = {
@@ -147,7 +148,7 @@ fun SearchScreen(
     filters: Set<SearchFilter> = ALL_FILTERS,
     onToggleFilter: (SearchFilter) -> Unit = {},
     coverArt: (String?, Boolean) -> CoverArtRef? = { _, _ -> null },
-    artistCovers: suspend (String) -> List<CoverArtRef> = { emptyList() },
+    artistArt: (Artist) -> ArtistArt = { ArtistArt() },
     playingSongId: String? = null,
     onBack: () -> Unit = {},
     onAlbumClick: (Album) -> Unit = {},
@@ -217,7 +218,7 @@ fun SearchScreen(
                     contentPadding = PaddingValues(bottom = 24.dp),
                 ) {
                     if (showPlaylists) playlistResults(results.playlists, playlistsLabel, coverArt, onPlaylistClick, onLongClick)
-                    if (showArtists) artistResults(results.artists, artistsLabel, coverArt, artistCovers, onArtistClick, onLongClick)
+                    if (showArtists) artistResults(results.artists, artistsLabel, artistArt, onArtistClick, onLongClick)
                     if (showAlbums) albumResults(results.albums, albumsLabel, coverArt, onAlbumClick, onLongClick)
                     if (showSongs) songResults(results.songs, songsLabel, coverArt, playingSongId, onSongClick, onLongClick)
                 }
@@ -310,21 +311,20 @@ private fun LazyListScope.albumResults(
 private fun LazyListScope.artistResults(
     artists: List<Artist>,
     header: String,
-    coverArt: (String?, Boolean) -> CoverArtRef?,
-    artistCovers: suspend (String) -> List<CoverArtRef>,
+    artistArt: (Artist) -> ArtistArt,
     onClick: (Artist) -> Unit,
     onLongClick: (MenuTarget) -> Unit,
 ) {
     if (artists.isEmpty()) return
     resultHeader(header)
     items(artists, key = { "artist-${it.id}" }) { artist ->
+        val art = artistArt(artist)
         ArtistListRow(
             artist = artist,
-            coverArt = coverArt,
-            artistCovers = artistCovers,
+            art = art,
             info = null,
             onClick = { onClick(artist) },
-            onLongClick = { onLongClick(MenuTarget.Artist(artist, coverArt(artist.coverArt, true))) },
+            onLongClick = { onLongClick(MenuTarget.Artist(artist, art.primary)) },
         )
     }
 }