feat(gpu): re-process trigger to apply new crop detectors to the existing library (#1202)

The siglip/ccip backfills skip images that already have current-version regions,
so adding crop detectors only affected NEW images — the back-catalogue would
never be re-cropped. Add a reprocess trigger that resets every done/error job of
a task back to pending, so the agent re-runs the FULL pipeline (figure detection
+ CCIP + concept/panel crops) over the whole library under the current detectors.

- reprocess_gpu_jobs(task='ccip') task + POST /api/gpu/reprocess.
- gpu store reprocess() + GpuAgentCard "Re-process library (re-detect + re-crop)"
  button with a confirm (it's heavy).
- Test: a done job resets to pending (attempts cleared).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/stores/gpu.js

@@ -29,5 +29,11 @@ export const useGpuStore = defineStore('gpu', () => {
     return await api.post('/api/gpu/backfill', { body: { task } })
   }
 
-  return { token, rotateToken, status, backfill }
+  // Reset every done/error `task` job to pending → re-run the WHOLE library
+  // under the current pipeline (e.g. after adding crop detectors).
+  async function reprocess(task = 'ccip') {
+    return await api.post('/api/gpu/reprocess', { body: { task } })
+  }
+
+  return { token, rotateToken, status, backfill, reprocess }
 })