address review: drop the dead disc query, pin the fetch bound, cover multi-page disc prune

app/src/main/kotlin/com/subtracks/data/db/LibraryDao.kt

@@ -82,9 +82,6 @@ interface LibraryDao {
         limit: Int,
     ): List<String>
 
-    @Query("SELECT albumId, disc FROM discs WHERE sourceId = :sourceId")
-    suspend fun discKeys(sourceId: Long): List<DiscKey>
-
     @Query(
         "SELECT albumId, disc FROM discs WHERE sourceId = :sourceId " +
             "AND (albumId > :afterAlbumId OR (albumId = :afterAlbumId AND disc > :afterDisc)) " +

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicSource.kt

@@ -282,11 +282,12 @@ class SubsonicSource(
     private companion object {
         const val PAGE_SIZE = 500
         const val MAX_PAGES = 20_000
-        const val MAX_CONCURRENT_FETCHES = 4
         const val THUMBNAIL_SIZE = 256
     }
 }
 
+internal const val MAX_CONCURRENT_FETCHES = 4
+
 private class PageCapExceeded(
     message: String,
 ) : SubsonicException(-1, message)