feat(ai): Phase 1C 管理看板 — 用量/成本/功能开关三合一

- UsageService 新增 get_daily_usage + aggregate_daily 日聚合能力
- 新增 3 个管理端点: /ai/admin/daily-usage, /ai/admin/flags (GET+POST)
- AiUsageDashboard 扩展为三 Tab: 用量概览/成本分析/功能开关
- 功能开关支持 Switch 实时切换，权限码 ai.admin.flags
- 日聚合用量 30 天趋势表，含 Token/成本汇总统计

apps/web/src/api/ai/usage.ts

@@ -45,6 +45,22 @@ export interface CostEstimate {
   currency: string;
 }
 
+export interface DailyUsageRow {
+  date: string;
+  feature: string;
+  provider: string;
+  model: string;
+  total_calls: number;
+  total_input_tokens: number;
+  total_output_tokens: number;
+  total_cost_cents: number;
+}
+
+export interface FeatureFlag {
+  feature: string;
+  is_enabled: boolean;
+}
+
 export const usageApi = {
   overview: async () => {
     const resp = await client.get('/ai/usage/overview');
@@ -74,4 +90,18 @@ export const usageApi = {
     const resp = await client.get('/ai/cost/estimate', { params });
     return resp.data.data as CostEstimate;
   },
+  getDailyUsage: async (startDate: string, endDate: string) => {
+    const resp = await client.get('/ai/admin/daily-usage', {
+      params: { start_date: startDate, end_date: endDate },
+    });
+    return resp.data.data as DailyUsageRow[];
+  },
+  getFeatureFlags: async () => {
+    const resp = await client.get('/ai/admin/flags');
+    return resp.data.data as FeatureFlag[];
+  },
+  updateFeatureFlag: async (feature: string, enabled: boolean) => {
+    const resp = await client.post('/ai/admin/flags', { feature, enabled });
+    return resp.data.data as { feature: string; enabled: boolean };
+  },
 };