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>

backend/app/main.py

@@ -371,7 +371,7 @@ async def root():
 
 
 # Include V2 API routers
-from app.routers import auth, tasks, admin
+from app.routers import auth, tasks, admin, translate
 from fastapi import UploadFile, File, Depends, HTTPException, status
 from sqlalchemy.orm import Session
 import hashlib
@@ -385,6 +385,7 @@ from app.services.task_service import task_service
 app.include_router(auth.router)
 app.include_router(tasks.router)
 app.include_router(admin.router)
+app.include_router(translate.router)
 
 
 # File upload endpoint