feat: notifications say what changed and alert once: a grouped notice names the tracks, grows without re-alerting, and a phone pile lists its titles (M542 #6146-#6151)

- An update to a grouped notice keeps its arrival time and email state, so the
  phone and the email no longer go off again each time it grows.
- Notices carry the names of what changed (newest ten), shown as three and
  "and N more": tracks still missing after the scan's move/replacement fold,
  duplicate groups new to a sweep, songs merged automatically, and the track
  and listener a playback error was for.
- Duplicates found counts only the groups new to that sweep; playback errors
  count reports, not the unresolved total.
- A scan failure says what stopped and links to Admin > Library.
- Android: a pile of more than three alerts lists the newest five titles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_sweep.go

@@ -112,25 +112,26 @@ func runDuplicateSweep(
 	return res, runErr
 }
 
-// notifyNewDuplicates tells admins when a sweep proposed a group it had not
-// proposed before (M489), counting every proposal awaiting review. A sweep
-// that only re-finds known groups says nothing, so reading the notice once
-// is enough until something new turns up.
+// notifyNewDuplicates tells admins when a sweep proposed groups it had not
+// proposed before (M489), counting and naming those (M542). A sweep that only
+// re-finds known groups says nothing, so reading the notice once is enough
+// until something new turns up.
 func notifyNewDuplicates(ctx context.Context, q *dbq.Queries, sweepStarted pgtype.Timestamptz, logger *slog.Logger) {
-	fresh, err := q.CountDuplicateGroupsDetectedSince(ctx, sweepStarted)
+	fresh, err := q.ListDuplicateGroupsDetectedSince(ctx, dbq.ListDuplicateGroupsDetectedSinceParams{
+		Since: sweepStarted, N: notifyItems,
+	})
 	if err != nil {
-		logger.Warn("duplicate sweep: counting new proposals failed", "err", err)
+		logger.Warn("duplicate sweep: listing new proposals failed", "err", err)
 		return
 	}
-	if fresh == 0 {
+	if len(fresh) == 0 {
 		return
 	}
-	pending, err := q.CountPendingDuplicateGroups(ctx)
-	if err != nil {
-		logger.Warn("duplicate sweep: counting pending proposals failed", "err", err)
-		return
+	items := make([]string, 0, len(fresh))
+	for _, g := range fresh {
+		items = append(items, TrackLabel(g.ArtistName, g.Title))
 	}
-	notifyAdmins(ctx, notifications.KindDuplicatesFound, notifications.Payload{Count: pending})
+	notifyAdmins(ctx, notifications.KindDuplicatesFound, notifications.Payload{Count: fresh[0].Total, Items: items})
 }
 
 func sweepDuplicates(