feat: add translation billing stats and remove Export/Settings pages

- Add TranslationLog model to track translation API usage per task
- Integrate Dify API actual price (total_price) into translation stats
- Display translation statistics in admin dashboard with per-task costs
- Remove unused Export and Settings pages to simplify frontend
- Add GET /api/v2/admin/translation-stats endpoint

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -5,9 +5,7 @@ import { apiClientV2 } from '@/services/apiV2'
 import LanguageSwitcher from '@/components/LanguageSwitcher'
 import {
   Upload,
-  Settings,
   FileText,
-  Download,
   Activity,
   LogOut,
   LayoutDashboard,
@@ -41,8 +39,6 @@ export default function Layout() {
     { to: '/processing', label: t('nav.processing'), icon: Activity, description: '處理進度', adminOnly: false },
     { to: '/results', label: t('nav.results'), icon: FileText, description: '查看結果', adminOnly: false },
     { to: '/tasks', label: '任務歷史', icon: History, description: '查看任務記錄', adminOnly: false },
-    { to: '/export', label: t('nav.export'), icon: Download, description: '導出文件', adminOnly: false },
-    { to: '/settings', label: t('nav.settings'), icon: Settings, description: '系統設定', adminOnly: false },
     { to: '/admin', label: '管理員儀表板', icon: Shield, description: '系統管理', adminOnly: true },
   ]