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/notifications/notifier.go

@@ -57,8 +57,9 @@ func New(db dbq.DBTX, bus *eventbus.Bus, logger *slog.Logger) *Notifier {
 // then nudges their open clients.
 //
 // For a coalescing kind, an unread row of that kind is updated in place
-// rather than a new one added. payload["count"] then adds to the unread
-// row's count for kinds whose events each mean "N more".
+// rather than a new one added: payload["count"] adds to the row's count and
+// payload["items"] joins its names. The row keeps the time it arrived, so an
+// update is not announced again.
 //
 // The error is for logging. A notification must never fail the action that
 // caused it, so producers log and carry on; NotifyLogged does exactly that.
@@ -183,7 +184,6 @@ func write(ctx context.Context, q *dbq.Queries, kind Kind, userID pgtype.UUID, b
 		Payload:     body,
 		CoalesceKey: &key,
 		EmailWanted: emailWanted,
-		SumCount:    specs[kind].sumCount,
 	})
 	return err
 }