Add an admin-only endpoint that returns deployment-level chat usage
statistics over a time range. Returns chat counts, active users,
message counts by role, token usage totals, and per-status breakdowns.

Sub chats (spawned by subagent delegation) are counted separately via
total_sub_chats, but their messages and token usage are included in
the totals so real LLM consumption is not undercounted. Chat counts,
active users, and by_status only reflect top-level conversations.

Authorization uses ResourceDeploymentConfig + ActionRead, matching
the existing chat admin endpoints.