fix(gpu-jobs): end the error-tombstone loop — deliberate retry semantics + poison-job guards

The hourly ccip backfill's skip-list lacked 'error' (and the daily
siglip/embed variants re-gated failures on their missing results), so every
permanently-bad file got a fresh doomed job each run — ~24 duplicate error
rows/day per file, the perpetual 'unprocessable' flood. An errored job is now
a TOMBSTONE: no backfill re-enqueues it; retry is deliberate-only via
/retry_errors (an errored back-catalogue needs one button press after a
model swap).

One shared set of dedupe DELETEs (services/ml/gpu_jobs.error_dedupe_statements)
runs before every backfill and inside /retry_errors: error rows made moot by a
later pending/leased/done row go first, then older duplicates (newest reason
survives) — so the error count reads as distinct failing files and a retry
can't fan one file out into duplicate pending jobs. /retry_errors now returns
{requeued, pruned} and the toast shows both.

Poison-loop guards (release and lease-expiry burn no attempts, so a job that
stalls its transfer or crashes the agent every time cycled forever —
operator-observed jobs 99044/125288/131594/143131):
- agent: 3 in-session transient bounces (fetch or submit) → fail with the real
  reason instead of another release; strikes never count while stopping, and
  clear on submit success. Agent build 2026-07-02.3.
- server: the 60s orphan sweep (statements shared between the beat task and
  GpuJobService so they can't drift) converts expired leases with >=5 lease
  grants and pending jobs with >=10 to 'error', preserving the last stored
  failure reason. Backstops old agent builds.

Tests: tombstone rule across all three backfill variants, moot-row pruning,
poison conversions, and the extended /retry_errors dedupe contract.

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 +337,9 @@ async function onBackfillSiglip() {
 async function onRetryErrors() {
   retrying.value = true
   try {
-    const { requeued } = await store.retryErrors()
-    toast({ text: `Requeued ${requeued} errored job${requeued === 1 ? '' : 's'} — run the agent to process them`, type: 'success' })
+    const { requeued, pruned } = await store.retryErrors()
+    const extra = pruned ? ` (${pruned} stale duplicate${pruned === 1 ? '' : 's'} pruned)` : ''
+    toast({ text: `Requeued ${requeued} errored job${requeued === 1 ? '' : 's'}${extra} — run the agent to process them`, type: 'success' })
     await refreshQueue()
   } catch (e) {
     toast({ text: `Could not retry errored jobs: ${e.message}`, type: 'error' })