feat(gpu): fast orphan recovery — graceful release + 60s sweep (#114)

So work an agent orphaned gets picked back up quickly, three layers:
- GpuJobService.release(): a graceful agent stop hands its still-leased jobs back
  to pending instantly (POST /api/gpu/jobs/release), no waiting out the lease.
- GpuJobService.recover_orphaned() + recover_orphaned_gpu_jobs Celery task on a
  60s beat: resets expired leases (a hard-crashed agent) to pending and keeps the
  queue counts honest even when nothing is leasing.
- Lease TTL 300→180s: still well above any single job (a capped-frame video embed
  is tens of seconds, and a live worker heartbeats), but a hard crash recovers
  faster once the sweep fires.

Tests: release returns-to-pending (token-scoped), recover_orphaned resets only
expired leases, release API round-trip.

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

backend/app/api/gpu.py

@@ -196,3 +196,18 @@ async def fail():
         )
         await session.commit()
     return jsonify({"ok": ok})
+
+
+@gpu_bp.route("/jobs/release", methods=["POST"])
+async def release():
+    """Graceful stop: the agent hands its still-leased jobs back to pending so
+    they're picked up immediately instead of waiting out the lease."""
+    body = await request.get_json(silent=True) or {}
+    agent_id = str(body.get("agent_id") or "agent")
+    job_ids = [int(x) for x in (body.get("job_ids") or [])]
+    async with get_session() as session:
+        if not await _agent_authed(session):
+            return jsonify({"error": "unauthorized"}), 401
+        n = await GpuJobService(session).release(agent_id, job_ids)
+        await session.commit()
+    return jsonify({"released": n})