feat(gpu): "Retry errored jobs" — scoped requeue of errors only

After an agent-side fix (e.g. the short-video sampler), the errored jobs
(~2.8k) have exhausted their 3 attempts and stay parked: backfill skips
images that already have a job, and /reprocess is the nuclear option (it
resets the 179k DONE jobs too). There was no way to re-run just the errors.

POST /api/gpu/retry_errors resets every status='error' job (all task types)
to pending with attempts=0 and the stored error cleared — a small inline
UPDATE that returns {requeued: n} so the UI toast can show the count.

UI: a "Retry errored jobs" button on the GPU-agent card, right under the
queue tiles; disabled when errored==0. With the agent now logging ffmpeg's
stderr on failure, retrying also reveals which errors were real vs victims
of the fps-filter bug.

Test: retry_errors requeues the errored job (fresh attempts, error cleared)
and leaves done work untouched; asserts via column selects (Core-DML
gotcha), not ORM refresh.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_api_gpu.py

@@ -1,7 +1,8 @@
 """GPU-job HTTP API (#114): bearer auth + lease/submit round-trip + backfill."""
 import pytest
+from sqlalchemy import select
 
-from backend.app.models import ImageRecord
+from backend.app.models import GpuJob, ImageRecord
 from backend.app.services.ml.gpu_jobs import GpuJobService
 from backend.app.services.ml.regions import RegionService
 
@@ -148,3 +149,39 @@ async def test_release_hands_job_back_to_pending(client, db):
     assert resp.status_code == 200 and (await resp.get_json())["released"] == 1
     st = await (await client.get("/api/gpu/status")).get_json()
     assert st["pending"] == 1 and st["leased"] == 0
+
+
+@pytest.mark.asyncio
+async def test_retry_errors_requeues_only_errored(client, db):
+    """/retry_errors resets ERRORED jobs (any task) to pending with a fresh
+    retry budget — and leaves done work untouched (it is NOT /reprocess)."""
+    img1 = await _img(db, "1" * 64)
+    img2 = await _img(db, "2" * 64)
+    svc = GpuJobService(db)
+    j_err = await svc.enqueue(img1.id, "ccip")
+    j_done = await svc.enqueue(img2.id, "siglip")
+    err_id = j_err.id
+    done_id = j_done.id
+    j_err.status = "error"
+    j_err.attempts = 3
+    j_err.error = "no frames sampled from video (unprocessable)"
+    j_done.status = "done"
+    await db.commit()
+
+    resp = await client.post("/api/gpu/retry_errors")
+    assert resp.status_code == 200
+    assert (await resp.get_json())["requeued"] == 1
+
+    # Column selects, not ORM refresh — the route wrote via Core DML.
+    row = (await db.execute(
+        select(GpuJob.status, GpuJob.attempts, GpuJob.error)
+        .where(GpuJob.id == err_id)
+    )).one()
+    assert tuple(row) == ("pending", 0, None)
+    done_status = await db.scalar(
+        select(GpuJob.status).where(GpuJob.id == done_id)
+    )
+    assert done_status == "done"
+
+    st = await (await client.get("/api/gpu/status")).get_json()
+    assert st["pending"] == 1 and st["error"] == 0