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/services/apiV2.ts

@@ -27,6 +27,7 @@ import type {
   TopUser,
   AuditLogListResponse,
   UserActivitySummary,
+  TranslationStats,
   ProcessingOptions,
   ProcessingMetadata,
   DocumentAnalysisResponse,
@@ -621,6 +622,14 @@ class ApiClientV2 {
     return response.data
   }
 
+  /**
+   * Get translation statistics (admin only)
+   */
+  async getTranslationStats(): Promise<TranslationStats> {
+    const response = await this.client.get<TranslationStats>('/admin/translation-stats')
+    return response.data
+  }
+
   // ==================== Translation APIs ====================
 
   /**