Add domain-scoped resource utilization metrics for usage reports

Follow-up from the usage-report wayfinder map FR-3620 (data-source decision FR-3623).

The agent-exported Prometheus gauge `backendai_container_utilization` carries `agent_id / kernel_id / session_id / user_id / project_id / value_type` labels but **no domain label**, and the allocation REST endpoints are superadmin+project-keyed. As a result the WebUI's weekly/monthly usage report ships **global scope only** for admins in v1; domain-scoped admin reports are deferred until the manager can serve them.

Needed (either path):

- a `domain` label on the utilization exporter, or
- a domain-scoped GraphQL utilization/stats field with domain-admin RBAC.

When this lands, the WebUI report view gains a domain scope parameter (phase 2 of the report feature).

JIRA Issue: FR-3628

Start with the agent-exported backend_ai_container_utilization gauge, the allocation REST endpoints, and the WebUI weekly/monthly usage report. Compare the two proposed paths: adding a domain label or exposing a domain-scoped GraphQL utilization/stats field with domain-admin RBAC. Done means the backend serves domain-scoped data and the report view can request a domain scope in phase 2.

Written by the indexing model from the issue text.