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/db/queries/tracks.sql

@@ -335,3 +335,14 @@ UPDATE tracks
    SET source_verdict = sqlc.arg(verdict)::text, source_verdict_at = now()
  WHERE id = sqlc.arg(id)
    AND source_verdict IS NOT NULL;
+
+-- name: ListStillMissingTrackLabels :many
+-- Which of the tracks a scan marked missing still are once it has adopted
+-- moves and folded replacements (M542): the ones worth telling an admin
+-- about, by name. A quality upgrade folds into its replacement and drops out.
+SELECT ar.name AS artist_name, t.title
+  FROM tracks t
+  JOIN artists ar ON ar.id = t.artist_id
+ WHERE t.id = ANY(sqlc.arg(ids)::uuid[])
+   AND t.missing_since IS NOT NULL
+ ORDER BY ar.name, t.title;