feat(grafana): add gpu workload history deep-link endpoint (#824)

[SekiXu]

What type of PR is this?

Feature — new backend API endpoint (Grafana deep-link for GPU workload history).

Which issue(s) this PR fixes?

Backend track of #824 and #825 (cross-track User Stories; this PR is the backend portion only — not closing the stories).

What this PR does?

Adds GET /api/v1/datacenters/{dataCenter}/grafana/gpuWorkloadHistory/{hostname}, returning the Device dashboard (UID i-device) deep-links for a physical node's GPU Utilization (panel 50) and VRAM (panel 51), so the Frontend can add a "View Workload History" button on the physical GPU table.

Response:

{
  "gpuUtilizationUrl": "https://<vip>/grafana/d/i-device/device?orgId=1&var-GPU_HOST=<host>&from=now-3h&to=now&viewPanel=50",
  "vramUrl":           "https://<vip>/grafana/d/i-device/device?orgId=1&var-GPU_HOST=<host>&from=now-3h&to=now&viewPanel=51",
  "enabled": true
}

Changes (additive, zero breaking change):

• definition/v1/grafana: new GpuWorkloadHistory struct (two URLs + enabled).
• handlers/grafana/links.go: genGpuUtilizationHistoryLink / genGpuVramHistoryLink (reuse existing link-gen style + base.DataCenterVip).
• handlers/grafana/handlers.go: forwardGpuWorkloadHistoryLinks + route.

Design notes:

• One endpoint returns both URLs (per the spike) → Frontend gets both in one call.
• Filter variable is var-GPU_HOST (hidden $GPU_HOST), not var-HOST — var-HOST resolves to ipmi_sensor.hostname → blank panels.
• enabled = nodes.IsExist(hostname) (cluster-wide). GetNodeGpusMap is intentionally avoided: it runs hex_sdk locally and reports the wrong node for remote hostnames.
• Auth-free (GET /grafana/* is in the API auth-free allowlist), consistent with the other Grafana link endpoints.
• Contract (see #824/#825): panel ids 50=Util / 51=VRAM and var-GPU_HOST must stay stable.

Test results (optional)

1). make sure the api docs have been updated

✅ Added /grafana/gpuWorkloadHistory/{hostname} to the OpenAPI spec
(cube-cos-openapi#103), and bumped the submodule pointer in this PR.

2). make sure the api works properly

• Built on the x86 build container (go 1.24.2); ELF x86-64.
• Deployed to test node sky150 and smoke-verified:
  • valid node → gpuUtilizationUrl (viewPanel=50) + vramUrl (viewPanel=51) + enabled:true
  • non-existent hostname → enabled:false
  • hostname consistency confirmed: gpu.host.host = Node.Hostname = sky150

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