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/dbq/playback_errors.sql.go

@@ -11,17 +11,6 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
-const countUnresolvedPlaybackErrors = `-- name: CountUnresolvedPlaybackErrors :one
-SELECT count(*)::bigint FROM playback_errors WHERE resolved_at IS NULL
-`
-
-func (q *Queries) CountUnresolvedPlaybackErrors(ctx context.Context) (int64, error) {
-	row := q.db.QueryRow(ctx, countUnresolvedPlaybackErrors)
-	var column_1 int64
-	err := row.Scan(&column_1)
-	return column_1, err
-}
-
 const insertPlaybackError = `-- name: InsertPlaybackError :one
 INSERT INTO playback_errors (track_id, user_id, client_id, kind, detail)
 VALUES ($1, $2, $3, $4, $5)