fix(tests): query admin_audit by its real timestamp column 'at'

Two requeue-audit tests in api_admin_crawler ordered by created_at, but
the admin_audit table's timestamp column is 'at' (migration 0019), so the
queries failed at runtime with 42703 (column does not exist). This was the
last pre-0.81 test breakage gating deploy; full suite now green locally
(cargo test --no-fail-fast).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/api_admin_crawler.rs

@@ -573,7 +573,7 @@ async fn requeue_chapter_audit_records_chapter_target_id(pool: PgPool) {
     let (target_kind, target_id): (String, Option<Uuid>) = sqlx::query_as(
         "SELECT target_kind, target_id FROM admin_audit \
          WHERE action = 'crawler_dead_jobs_requeue' \
-         ORDER BY created_at DESC LIMIT 1",
+         ORDER BY at DESC LIMIT 1",
     )
     .fetch_one(&pool)
     .await
@@ -601,7 +601,7 @@ async fn requeue_all_audit_omits_target_id_but_logs_count(pool: PgPool) {
         sqlx::query_as(
             "SELECT target_kind, target_id, payload FROM admin_audit \
              WHERE action = 'crawler_dead_jobs_requeue' \
-             ORDER BY created_at DESC LIMIT 1",
+             ORDER BY at DESC LIMIT 1",
         )
         .fetch_one(&pool)
         .await