feat: build out centralcloud_ops staff portal with Grafana OnCall backend
- Add CentralcloudCore.OnCall API client (alert groups, schedules, users,
escalation chains) — talks to Grafana OnCall HTTP API v1
- Add centralcloud_ops application, endpoint, router, layouts
- Add RequireStaff auth plug, SessionController, HealthController
- Add DashboardLive: firing/acked alerts with ack/resolve actions, auto-refresh
- Add IncidentsLive: filterable incident list by status
- Add IncidentLive: incident detail with ack/resolve/silence actions
- Add OnCallLive: schedule cards showing who is currently on-call
- Add StakeholdersLive: HostBill client search + service view + comms panel
- Wire ONCALL_URL / ONCALL_API_TOKEN env vars in config and runtime.exs
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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