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.

backend/app/celery_signals.py

@@ -54,7 +54,14 @@ _INT32_MIN = -2_147_483_648
 
 def _queue_for(task) -> str:
     """Reverse the task→queue routing from celery_app.task_routes.
-    Keep in sync if task_routes is reordered."""
+    Keep in sync if task_routes is reordered.
+
+    Audit 2026-06-02: backup/admin/library_audit prefixes were
+    missing here even though task_routes sent all three to
+    'maintenance'. The TaskRun.queue column then lied for those
+    rows (claimed 'default') so per-queue dashboard filters and
+    per-queue threshold overrides silently missed them.
+    """
     name = getattr(task, "name", "") or ""
     if name.startswith("backend.app.tasks.import_file."):
         return "import"
@@ -66,7 +73,12 @@ def _queue_for(task) -> str:
         return "download"
     if name.startswith("backend.app.tasks.scan."):
         return "scan"
-    if name.startswith("backend.app.tasks.maintenance."):
+    if name.startswith((
+        "backend.app.tasks.maintenance.",
+        "backend.app.tasks.backup.",
+        "backend.app.tasks.admin.",
+        "backend.app.tasks.library_audit.",
+    )):
         return "maintenance"
     return "default"