feat(dashboard): add per-user drill-down for group, model, and endpoint distributions

Click on a group name, model name, or endpoint name in the distribution
tables to expand and show per-user usage breakdown (requests, tokens,
actual cost, standard cost).

Backend: new GET /admin/dashboard/user-breakdown API with group_id,
model, endpoint, endpoint_type filters.
Frontend: clickable rows with expand/collapse sub-table in all three
distribution charts.

frontend/src/types/index.ts

@@ -1202,6 +1202,15 @@ export interface GroupStat {
   actual_cost: number // 实际扣除
 }
 
+export interface UserBreakdownItem {
+  user_id: number
+  email: string
+  requests: number
+  total_tokens: number
+  cost: number
+  actual_cost: number
+}
+
 export interface UserUsageTrendPoint {
   date: string
   user_id: number