Add missing filters, orderings and columns to the Agent list and schedulable count to the dashboard

Page: /admin/agent (Agent tab) + /admin/dashboard AgentStats. Source: report-resources.md.

**AgentList** (agent_nodes graphene, all <=26.4.0):

- Filters: scaling_group, region, version, first_contact (datetime), status_changed, lost_at.
- Orderings: id (column dataIndex is row_id -> sortKey id), region, version, lost_at.
- Columns: restore `architecture` (BAIAgentTable builds it, AgentList drops it), add lost_at (hidden by default).
- Do NOT add available_slots/occupied_slots ordering (removed on backend main, #13922).

**Dashboard AgentStats**: show `schedulable / total` alive agent counts next to the existing resource totals using two aliased count-only `agent_nodes(first: 1) { count `} selections (ALIVE, ALIVE & schedulable). No version gate needed. Origin: Teams request.

JIRA Issue: FR-3909

Start with report-resources.md and the AgentList and AgentStats entry points for /admin/agent and /admin/dashboard; compare AgentList with BAIAgentTable for the missing architecture column. Verify the requested filters, orderings, columns, and aliased ALIVE and schedulable count selections, then confirm the dashboard shows schedulable/total alive agents.

Written by the indexing model from the issue text.