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/scanner.go

@@ -83,6 +83,10 @@ type Stats struct {
 	// Only a full Scan sets these — see reconcileMissing.
 	Missing  int `json:"missing"`
 	Restored int `json:"restored"`
+	// marked are the tracks this scan's reconcile marked missing, so the scan
+	// can tell admins which of them are still gone once it has adopted moves
+	// and folded replacements (M542).
+	marked []pgtype.UUID
 }
 
 type Scanner struct {
@@ -198,6 +202,9 @@ func (s *Scanner) Scan(ctx context.Context, progressCb func(Stats)) (Stats, erro
 	if _, err := PairMissingTracks(ctx, s.pool, s.logger); err != nil && ctx.Err() == nil {
 		s.logger.Warn("library scan: missing-pair pass failed", "err", err)
 	}
+	// Told only now, so a file that moved or was upgraded, which this scan
+	// marked and then healed, is not reported as lost.
+	s.notifyStillMissing(ctx, stats.marked)
 	return stats, nil
 }