library: recompute artist aggregates at the sync boundary

Replace the per-credit invalidation with one signal from the sync's write
transactions: if artists, albums or songs changed, the source is marked stale
and a single aggregate pass refreshes counts, plays and the credited flag in
one query. Identical syncs no longer run any of those scans, and a new credit
path cannot forget to invalidate the cache.

A changed sync still reads the credit unions once instead of separately for
counts and plays, and migration 33->34 remains a frozen snapshot rather than
sharing the runtime query text.

app/src/test/kotlin/com/subtracks/data/db/LibraryDaoTest.kt

@@ -189,8 +189,7 @@ class LibraryDaoTest {
                 ),
             )
 
-            dao.recomputeAlbumPlayData(sourceId)
-            dao.recomputeArtistPlayData(sourceId)
+            dao.recomputeAggregates(sourceId)
 
             assertEquals(3L, dao.albumOnce(sourceId, "al-1")!!.playCount)
             assertEquals(100L, dao.albumOnce(sourceId, "al-1")!!.played)

app/src/test/kotlin/com/subtracks/data/db/QueryTimingMeasurementTest.kt

@@ -134,7 +134,7 @@ class QueryTimingMeasurementTest {
         timed("home recently starred") { dao.recentlyStarredSongs(1, 20).first() }
         timed("home genres") { dao.genresByMostPlayed(1).first() }
         timed("home decades") { dao.decades(1).first() }
-        timed("recompute play data") { dao.recomputePlayData(1) }
+        timed("recompute aggregates") { dao.recomputeAggregates(1) }
     }
 
     private suspend fun <T : Any> PagingSource<Int, T>.firstPage(): List<T> =