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>

openspec/specs/result-export/spec.md

@@ -114,3 +114,57 @@ The system SHALL provide export formats that preserve document structure for dow
 - **AND** identify layout regions (header, body, footer, sidebar)
 - **AND** provide confidence scores for layout detection
 
+### Requirement: Translation Result JSON Export
+
+The system SHALL support exporting translation results as independent JSON files following a defined schema.
+
+#### Scenario: Export translation result JSON
+- **WHEN** translation completes for a document
+- **THEN** system SHALL save translation to `{filename}_translated_{lang}.json`
+- **AND** file SHALL be stored alongside original `{filename}_result.json`
+- **AND** original result file SHALL remain unchanged
+
+#### Scenario: Translation JSON schema compliance
+- **WHEN** translation result is saved
+- **THEN** JSON SHALL include schema_version field ("1.0.0")
+- **AND** SHALL include source_document reference
+- **AND** SHALL include source_lang and target_lang
+- **AND** SHALL include provider identifier (e.g., "dify")
+- **AND** SHALL include translated_at timestamp
+- **AND** SHALL include translations dict mapping element_id to translated content
+
+#### Scenario: Translation statistics in export
+- **WHEN** translation result is saved
+- **THEN** JSON SHALL include statistics object with:
+  - total_elements: count of all elements in document
+  - translated_elements: count of successfully translated elements
+  - skipped_elements: count of non-translatable elements (images, charts, etc.)
+  - total_characters: character count of translated text
+  - processing_time_seconds: translation duration
+
+#### Scenario: Table cell translation in export
+- **WHEN** document contains tables
+- **THEN** translation JSON SHALL represent table translations as:
+  ```json
+  {
+    "table_1_0": {
+      "cells": [
+        {"row": 0, "col": 0, "content": "Translated cell text"},
+        {"row": 0, "col": 1, "content": "Another cell"}
+      ]
+    }
+  }
+  ```
+- **AND** row/col positions SHALL match original table structure
+
+#### Scenario: Download translation result via API
+- **WHEN** GET request to `/api/v2/translate/{task_id}/result?lang={lang}`
+- **THEN** system SHALL return translation JSON content
+- **AND** Content-Type SHALL be application/json
+- **AND** response SHALL include appropriate cache headers
+
+#### Scenario: List available translations
+- **WHEN** GET request to `/api/v2/tasks/{task_id}/translations`
+- **THEN** system SHALL return list of available translation languages
+- **AND** include translation metadata (translated_at, provider, statistics)
+