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/scanrun.go

@@ -31,6 +31,13 @@ type LibraryStageTallies struct {
 	Restored int `json:"restored"`
 }
 
+func (st Stats) tallies() LibraryStageTallies {
+	return LibraryStageTallies{
+		Scanned: st.Scanned, Added: st.Added, Updated: st.Updated, Skipped: st.Skipped,
+		Errored: st.Errored, Missing: st.Missing, Restored: st.Restored,
+	}
+}
+
 // MBIDBackfillStageTallies wires BackfillMBIDsResult into the scan_runs jsonb column.
 type MBIDBackfillStageTallies struct {
 	Processed  int `json:"processed"`
@@ -105,7 +112,7 @@ func RunScan(
 		var lastStats Stats
 		libPub := newStagePublisher(ctx, 500, time.Second,
 			func() []byte {
-				blob, _ := json.Marshal(LibraryStageTallies(lastStats))
+				blob, _ := json.Marshal(lastStats.tallies())
 				return blob
 			},
 			func(blob []byte) error {