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/GpuAgentCard.vue

@@ -337,8 +337,11 @@ async function onBackfillSiglip() {
 async function onRetryErrors() {
   retrying.value = true
   try {
-    const { requeued, pruned } = await store.retryErrors()
-    const extra = pruned ? ` (${pruned} stale duplicate${pruned === 1 ? '' : 's'} pruned)` : ''
+    const { requeued, pruned, defects_kept: kept } = await store.retryErrors()
+    const extras = []
+    if (pruned) extras.push(`${pruned} stale duplicate${pruned === 1 ? '' : 's'} pruned`)
+    if (kept) extras.push(`${kept} known-bad file${kept === 1 ? '' : 's'} kept for recovery`)
+    const extra = extras.length ? ` (${extras.join(', ')})` : ''
     toast({ text: `Requeued ${requeued} errored job${requeued === 1 ? '' : 's'}${extra} — run the agent to process them`, type: 'success' })
     await refreshQueue()
   } catch (e) {