fix(audit-g4): status-enum miss batch

Five extension-miss findings from the 2026-06-02 audit, where a status
value was added on one side but a downstream consumer didn't pick it up.

- download_service._phase3_persist: explicit branches for
  ImportResult.status in ('failed','refreshed'). For 'failed' (archive
  probe crash from _import_archive), unlink the source file so the
  filesystem scanner doesn't re-import and re-crash on the same
  archive forever. 'refreshed' is currently unreachable from the
  download path (no deep=True) but matches the importer's documented
  contract; treat as 'attached'.

- gallery-dl backfill auto-complete now gates on dl_result.success +
  no error_type, not just return_code==0 + files_downloaded==0.
  VALIDATION_FAILED exits the subprocess with returncode=0 and
  files_downloaded=0 when every file was quarantined, matching the
  prior predicate exactly and zeroing the operator's armed backfill
  budget on the FIRST quarantine run instead of decrementing.

- attach_in_place archive dispatch now threads artist + source_row
  through _import_archive (and _import_media for archive members)
  and _supersede. The path-walk fallback (_resolve_artist) is still
  used by filesystem-import; the download path now binds
  ImageProvenance to the explicit subscription Source instead of
  rediscovering by (artist_id, platform).

- Three FE handlers now recognize status:'deferred' from
  /api/sources/<id>/check: SubscriptionsTab.onCheck (was toasting
  "event #undefined"), SubscriptionsTab.checkAll (was counting
  deferred as queued), DownloadEventRow.onRetry (was saying
  "re-queued" when nothing was). Pattern matches DownloadsTab.onRetryAll
  which already had it.

- celery_signals._queue_for now maps backup/admin/library_audit
  prefixes to 'maintenance' (matching task_routes). TaskRun.queue
  was returning 'default' for those rows, so per-queue dashboard
  filters and per-queue threshold overrides (added in G3) silently
  missed them.

frontend/src/components/downloads/DownloadEventRow.vue

@@ -124,10 +124,16 @@ async function onRetry() {
   if (!props.event.source_id) return
   retrying.value = true
   try {
-    await sourcesStore.checkNow(props.event.source_id)
-    toast({
-      text: `Source check re-queued`, type: 'success',
-    })
+    const body = await sourcesStore.checkNow(props.event.source_id)
+    // Audit 2026-06-02: the previous handler unconditionally toasted
+    // "re-queued" even when the platform was in cooldown (202 +
+    // status='deferred'). Operator thought work was in flight when
+    // nothing was actually enqueued.
+    if (body?.status === 'deferred') {
+      toast({ text: 'Retry deferred — platform in cooldown', type: 'info' })
+    } else {
+      toast({ text: 'Source check re-queued', type: 'success' })
+    }
   } catch (e) {
     const isInFlight = !!e?.body?.download_event_id
     toast({