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/schemas/__init__.py

@@ -13,6 +13,16 @@ from app.schemas.task import (
     TaskStatsResponse,
     TaskStatusEnum,
 )
+from app.schemas.translation import (
+    TranslationStatusEnum,
+    TranslationRequest,
+    TranslationProgress,
+    TranslationStatusResponse,
+    TranslationStartResponse,
+    TranslationStatistics,
+    TranslationResultResponse,
+    TranslationListResponse,
+)
 
 __all__ = [
     # Auth
@@ -27,4 +37,13 @@ __all__ = [
     "TaskListResponse",
     "TaskStatsResponse",
     "TaskStatusEnum",
+    # Translation
+    "TranslationStatusEnum",
+    "TranslationRequest",
+    "TranslationProgress",
+    "TranslationStatusResponse",
+    "TranslationStartResponse",
+    "TranslationStatistics",
+    "TranslationResultResponse",
+    "TranslationListResponse",
 ]