feat: add translated PDF export with layout preservation

Adds the ability to download translated documents as PDF files while
preserving the original document layout. Key changes:

- Add apply_translations() function to merge translation JSON with UnifiedDocument
- Add generate_translated_pdf() method to PDFGeneratorService
- Add POST /api/v2/translate/{task_id}/pdf endpoint
- Add downloadTranslatedPdf() method and PDF button in frontend
- Add comprehensive unit tests (52 tests: merge, PDF generation, API endpoints)
- Archive add-translated-pdf-export proposal

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

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

openspec/specs/translation/spec.md

@@ -186,3 +186,74 @@ The system SHALL support common languages through DIFY AI service.
   - Vietnamese (vi)
   - Thai (th)
 
+### Requirement: Translated PDF Generation
+
+The system SHALL support generating PDF files with translated content while preserving the original document layout.
+
+#### Scenario: Generate translated PDF from Direct track document
+- **GIVEN** a completed translation for a Direct track processed document
+- **WHEN** user requests translated PDF via `POST /api/v2/translate/{task_id}/pdf?lang={target_lang}`
+- **THEN** the system loads the translation JSON file
+- **AND** merges translations with UnifiedDocument by element_id
+- **AND** generates PDF with translated text at original positions
+- **AND** returns PDF file with Content-Type `application/pdf`
+
+#### Scenario: Generate translated PDF from OCR track document
+- **GIVEN** a completed translation for an OCR track processed document
+- **WHEN** user requests translated PDF
+- **THEN** the system generates PDF preserving all OCR layout information
+- **AND** replaces original text with translated content
+- **AND** maintains table structure with translated cell content
+
+#### Scenario: Handle missing translations gracefully
+- **GIVEN** a translation JSON missing some element_id entries
+- **WHEN** generating translated PDF
+- **THEN** the system uses original content for missing translations
+- **AND** logs warning for each fallback
+- **AND** completes PDF generation successfully
+
+#### Scenario: Translated PDF for incomplete translation
+- **GIVEN** a task with translation status "pending" or "translating"
+- **WHEN** user requests translated PDF
+- **THEN** the system returns 400 Bad Request
+- **AND** includes error message indicating translation not complete
+
+#### Scenario: Translated PDF for non-existent translation
+- **GIVEN** a task that has not been translated to requested language
+- **WHEN** user requests translated PDF with `lang=fr`
+- **THEN** the system returns 404 Not Found
+- **AND** includes error message indicating no translation for language
+
+---
+
+### Requirement: Translation Merge Service
+
+The system SHALL provide a service to merge translation data with UnifiedDocument.
+
+#### Scenario: Merge text element translations
+- **GIVEN** a UnifiedDocument with text elements
+- **AND** a translation JSON with matching element_ids
+- **WHEN** applying translations
+- **THEN** the system replaces content field for each matched element
+- **AND** preserves all other element properties (bounding_box, style_info, etc.)
+
+#### Scenario: Merge table cell translations
+- **GIVEN** a UnifiedDocument containing table elements
+- **AND** a translation JSON with table_cell translations like:
+  ```json
+  {
+    "table_1_0": {
+      "cells": [{"row": 0, "col": 0, "content": "Translated"}]
+    }
+  }
+  ```
+- **WHEN** applying translations
+- **THEN** the system updates cell content at matching row/col positions
+- **AND** preserves cell structure and styling
+
+#### Scenario: Non-destructive merge operation
+- **GIVEN** a UnifiedDocument
+- **WHEN** applying translations
+- **THEN** the system creates a modified copy
+- **AND** original UnifiedDocument remains unchanged
+