feat: 관리자 대시보드 — 토큰·비용·응답시간 집계 API + 앱 /admin 페이지

- backend apps/stats: GET /api/v1/admin/stats?from&to → totals / byUser / byDay. ChatMessage(assistant) 집계만, is_superuser 아니면 403. 테스트 3개
- frontend features/admin: 요약 카드 4개(요청·토큰·비용·평균 응답), 일별 토큰 막대(div), 사용자별 표, 기간 프리셋(오늘/7/30/90일). 헤더에 ADMIN 만 아이콘. 테스트 2개

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

2_frontend/src/routes.tsx

@@ -12,6 +12,7 @@ const SessionListPage = lazy(() => import("@/features/snap/pages/SessionListPage
 const NewChatPage = lazy(() => import("@/features/snap/pages/NewChatPage"))
 const SessionChatPage = lazy(() => import("@/features/snap/pages/SessionChatPage"))
 const SnippetPalettePage = lazy(() => import("@/features/snippets/pages/SnippetPalettePage"))
+const AdminStatsPage = lazy(() => import("@/features/admin/pages/AdminStatsPage"))
 
 const fallback = <CenterSpinner />
 
@@ -35,6 +36,7 @@ export const routes: RouteObject[] = [
               { path: PATHS.SNAP.slice(1), element: wrap(<SessionListPage />) },
               { path: PATHS.SNAP_NEW.slice(1), element: wrap(<NewChatPage />) },
               { path: PATHS.SNAP_SESSION.slice(1), element: wrap(<SessionChatPage />) },
+              { path: PATHS.ADMIN.slice(1), element: wrap(<AdminStatsPage />) },
             ],
           },
           {