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

backend/app/api/gpu.py

@@ -96,6 +96,19 @@ async def backfill():
     return jsonify({"celery_task_id": r.id, "task": task}), 202
 
 
+@gpu_bp.route("/reprocess", methods=["POST"])
+async def reprocess():
+    """Reset every done/error job of `task` back to pending so the agent re-runs
+    the WHOLE library under the current pipeline (e.g. after adding crop
+    detectors). Heavy — the back-catalogue is otherwise skipped by the backfills."""
+    body = await request.get_json(silent=True) or {}
+    task = str(body.get("task") or "ccip")
+    from ..tasks.ml import reprocess_gpu_jobs
+
+    r = reprocess_gpu_jobs.delay(task)
+    return jsonify({"celery_task_id": r.id, "task": task}), 202
+
+
 # --- Agent (bearer token): lease / submit / heartbeat / fail ------------
 
 @gpu_bp.route("/jobs/lease", methods=["POST"])