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>

backend/app/api/gpu.py

@@ -11,7 +11,7 @@ homelab admin.
 import secrets
 
 from quart import Blueprint, jsonify, request
-from sqlalchemy import func, select
+from sqlalchemy import func, select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 
 from ..extensions import get_session
@@ -109,6 +109,27 @@ async def reprocess():
     return jsonify({"celery_task_id": r.id, "task": task}), 202
 
 
+@gpu_bp.route("/retry_errors", methods=["POST"])
+async def retry_errors():
+    """Requeue every ERRORED job (all task types) back to pending — the scoped
+    recovery after an agent-side fix (e.g. the short-video sampler), where
+    /reprocess would needlessly re-run the whole done library too. Attempts and
+    the stored error reset so each job gets its full retry budget under the
+    fixed pipeline. Small row count (errors only) → inline UPDATE, and the
+    response carries the number requeued for the UI toast."""
+    async with get_session() as session:
+        res = await session.execute(
+            update(GpuJob)
+            .where(GpuJob.status == "error")
+            .values(
+                status="pending", attempts=0, error=None, lease_token=None,
+                leased_at=None, lease_expires_at=None, updated_at=func.now(),
+            )
+        )
+        await session.commit()
+    return jsonify({"requeued": res.rowcount or 0})
+
+
 # --- Agent (bearer token): lease / submit / heartbeat / fail ------------
 
 @gpu_bp.route("/jobs/lease", methods=["POST"])