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/notify.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/jackc/pgx/v5/pgtype"
 
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/notifications"
 )
 
@@ -34,6 +35,42 @@ func notifyAdmins(ctx context.Context, kind notifications.Kind, p notifications.
 	n.NotifyLogged(ctx, kind, notifications.ToAdmins(pgtype.UUID{}), p.Map())
 }
 
+// notifyItems is how many names a notice carries; the notifications package
+// keeps the newest ten across a coalesced burst.
+const notifyItems = 10
+
+// TrackLabel names a track as a notice does: "Artist – Title", or the title
+// alone when the artist is unknown.
+func TrackLabel(artist, title string) string {
+	if artist == "" {
+		return title
+	}
+	return artist + " – " + title
+}
+
+// notifyStillMissing tells admins which of the tracks a scan marked missing
+// are still gone after it adopted moves and folded replacements (M542). A
+// marked track the scan healed is not news: reporting it is what made a
+// quality upgrade read as lost music.
+func (s *Scanner) notifyStillMissing(ctx context.Context, marked []pgtype.UUID) {
+	if len(marked) == 0 || ctx.Err() != nil {
+		return
+	}
+	rows, err := dbq.New(s.pool).ListStillMissingTrackLabels(ctx, marked)
+	if err != nil {
+		s.logger.Warn("library scan: listing missing tracks for the notice failed", "err", err)
+		return
+	}
+	if len(rows) == 0 {
+		return
+	}
+	items := make([]string, 0, min(len(rows), notifyItems))
+	for _, r := range rows[:min(len(rows), notifyItems)] {
+		items = append(items, TrackLabel(r.ArtistName, r.Title))
+	}
+	notifyAdmins(ctx, notifications.KindTracksMissing, notifications.Payload{Count: int64(len(rows)), Items: items})
+}
+
 // notifyScanFinished tells admins a scan run ended in error. A scan cut short
 // by shutdown is not a failure anyone needs telling about.
 func notifyScanFinished(ctx context.Context, errMsg string) {