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/duplicates.sql

@@ -184,15 +184,25 @@ UPDATE duplicate_groups
    SET status = 'dismissed', resolved_at = now()
  WHERE id = sqlc.arg(id) AND status = 'pending';
 
--- name: CountDuplicateGroupsDetectedSince :one
+-- name: ListDuplicateGroupsDetectedSince :many
 -- Pending proposals first made at or after `since`: what a sweep that started
--- then found for the first time. A refreshed proposal keeps its detected_at,
--- so a sweep that only re-finds known groups counts none (M489: admins are
--- told about new duplicates, not reminded of the same ones every sweep).
-SELECT count(*)::bigint
-  FROM duplicate_groups
- WHERE status = 'pending'
-   AND detected_at >= sqlc.arg(since);
+-- then found for the first time, each named by one of its songs, and `total`
+-- of them however many are listed. A refreshed proposal keeps its detected_at,
+-- so a sweep that only re-finds known groups lists none (M489: admins are told
+-- about new duplicates, not reminded of the same ones every sweep).
+SELECT ar.name AS artist_name, t.title, count(*) OVER ()::bigint AS total
+  FROM duplicate_groups g
+  JOIN LATERAL (SELECT m.track_id
+                  FROM duplicate_group_members m
+                 WHERE m.group_id = g.id
+                 ORDER BY m.track_id
+                 LIMIT 1) one ON true
+  JOIN tracks t ON t.id = one.track_id
+  JOIN artists ar ON ar.id = t.artist_id
+ WHERE g.status = 'pending'
+   AND g.detected_at >= sqlc.arg(since)
+ ORDER BY g.detected_at DESC, g.id
+ LIMIT sqlc.arg(n);
 
 -- name: ListDuplicateGroupsForResolve :many
 -- Every pending group's present copies, for the resolver (M498 #5435): what it