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/api/admin/dashboard.ts

@@ -12,6 +12,7 @@ import type {
   ApiKeyUsageTrendPoint,
   UserUsageTrendPoint,
   UserSpendingRankingResponse,
+  UserBreakdownItem,
   UsageRequestType
 } from '@/types'
 
@@ -156,6 +157,29 @@ export async function getGroupStats(params?: GroupStatsParams): Promise<GroupSta
   return data
 }
 
+export interface UserBreakdownParams {
+  start_date?: string
+  end_date?: string
+  group_id?: number
+  model?: string
+  endpoint?: string
+  endpoint_type?: 'inbound' | 'upstream' | 'path'
+  limit?: number
+}
+
+export interface UserBreakdownResponse {
+  users: UserBreakdownItem[]
+  start_date: string
+  end_date: string
+}
+
+export async function getUserBreakdown(params: UserBreakdownParams): Promise<UserBreakdownResponse> {
+  const { data } = await apiClient.get<UserBreakdownResponse>('/admin/dashboard/user-breakdown', {
+    params
+  })
+  return data
+}
+
 /**
  * Get dashboard snapshot v2 (aggregated response for heavy admin pages).
  */