feat: add document translation via DIFY AI API

Implement document translation feature using DIFY AI API with batch processing:

Backend:
- Add DIFY client with batch translation support (5000 chars, 20 items per batch)
- Add translation service with element extraction and result building
- Add translation router with start/status/result/list/delete endpoints
- Add translation schemas (TranslationRequest, TranslationStatus, etc.)

Frontend:
- Enable translation UI in TaskDetailPage
- Add translation API methods to apiV2.ts
- Add translation types

Features:
- Batch translation with numbered markers [1], [2], [3]...
- Support for text, title, header, footer, paragraph, footnote, table cells
- Translation result JSON with statistics (tokens, latency, batch_count)
- Background task processing with progress tracking

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

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

frontend/src/services/apiV2.ts

@@ -35,6 +35,11 @@ import type {
   DocumentAnalysisResponse,
   PreprocessingPreviewRequest,
   PreprocessingPreviewResponse,
+  TranslationRequest,
+  TranslationStartResponse,
+  TranslationStatusResponse,
+  TranslationListResponse,
+  TranslationResult,
 } from '@/types/apiV2'
 
 /**
@@ -613,6 +618,74 @@ class ApiClientV2 {
     )
     return response.data
   }
+
+  // ==================== Translation APIs ====================
+
+  /**
+   * Start a translation job
+   */
+  async startTranslation(taskId: string, request: TranslationRequest): Promise<TranslationStartResponse> {
+    const response = await this.client.post<TranslationStartResponse>(
+      `/translate/${taskId}`,
+      request
+    )
+    return response.data
+  }
+
+  /**
+   * Get translation status
+   */
+  async getTranslationStatus(taskId: string): Promise<TranslationStatusResponse> {
+    const response = await this.client.get<TranslationStatusResponse>(
+      `/translate/${taskId}/status`
+    )
+    return response.data
+  }
+
+  /**
+   * Get translation result
+   */
+  async getTranslationResult(taskId: string, lang: string): Promise<TranslationResult> {
+    const response = await this.client.get<TranslationResult>(
+      `/translate/${taskId}/result`,
+      { params: { lang } }
+    )
+    return response.data
+  }
+
+  /**
+   * Download translation result as JSON file
+   */
+  async downloadTranslation(taskId: string, lang: string): Promise<void> {
+    const response = await this.client.get(`/translate/${taskId}/result`, {
+      params: { lang },
+      responseType: 'blob',
+    })
+
+    const blob = new Blob([response.data], { type: 'application/json' })
+    const link = document.createElement('a')
+    link.href = window.URL.createObjectURL(blob)
+    link.download = `${taskId}_translated_${lang}.json`
+    link.click()
+    window.URL.revokeObjectURL(link.href)
+  }
+
+  /**
+   * List available translations for a task
+   */
+  async listTranslations(taskId: string): Promise<TranslationListResponse> {
+    const response = await this.client.get<TranslationListResponse>(
+      `/translate/${taskId}/translations`
+    )
+    return response.data
+  }
+
+  /**
+   * Delete a translation
+   */
+  async deleteTranslation(taskId: string, lang: string): Promise<void> {
+    await this.client.delete(`/translate/${taskId}/translations/${lang}`)
+  }
 }
 
 // Export singleton instance