feat(triage): failed-processing triage — probe errored files, flag defects, recover (#125 C1-C3)

An errored GPU job's stored reason is a suspicion; the file probe is the
verdict. A 15-min beat sweep (triage_gpu_errors) runs verify_integrity's own
probe (sha256 + decode) on each errored image ONCE and writes both verdicts:
ImageRecord.integrity_status and the new GpuJob.triage_status ('defect' |
'file_ok', migration 0072). Every classification logs at WARNING so it
surfaces in Logs/System Activity.

- 'defect' rows are excluded from /retry_errors (re-running a known-bad file
  burns agent time re-minting the tombstone); response now reports
  defects_kept and the GpuAgentCard toast says so.
- GET /api/gpu/errors: triage view — reason buckets (classify_reason),
  probe verdicts, per-job detail. POST /errors/triage runs the sweep now.
- POST /api/gpu/errors/<id>/recover: reuses the Layer-2 refetch pattern —
  delete the defective copy + record (full cascade takes the tombstones too)
  and re-poll its subscription Source so a fresh copy re-imports and re-enters
  the pipeline; 'no_source' when nothing pollable resolves.
- New 'Failed processing' card (GpuTriageCard) in Maintenance: verdict counts,
  reason summary, probe-now, defect list with thumbnails + per-image Recover.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/settings/MaintenancePanel.vue

@@ -13,6 +13,7 @@
         <ThumbnailBackfillCard />
         <ArchiveReextractCard />
         <MissingFileRepairCard />
+        <GpuTriageCard />
         <DbMaintenanceCard />
       </div>
     </section>
@@ -48,6 +49,7 @@ import MLBackfillCard from './MLBackfillCard.vue'
 import ThumbnailBackfillCard from './ThumbnailBackfillCard.vue'
 import ArchiveReextractCard from './ArchiveReextractCard.vue'
 import MissingFileRepairCard from './MissingFileRepairCard.vue'
+import GpuTriageCard from './GpuTriageCard.vue'
 import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import MLThresholdSliders from './MLThresholdSliders.vue'
 import HeadsCard from './HeadsCard.vue'