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>

backend/app/routers/translate.py

@@ -501,3 +501,139 @@ async def delete_translation(
     logger.info(f"Deleted translation {lang} for task {task_id}")
 
     return None
+
+
+@router.post("/{task_id}/pdf")
+async def download_translated_pdf(
+    task_id: str,
+    lang: str = Query(..., description="Target language code"),
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    Download a translated PDF with layout preservation.
+
+    - **task_id**: Task UUID
+    - **lang**: Target language code (e.g., 'en', 'ja')
+
+    Returns PDF file with translated content preserving original layout.
+    """
+    from app.services.pdf_generator_service import pdf_generator_service
+    from app.services.translation_service import list_available_translations
+    import tempfile
+
+    # Verify task ownership
+    task = task_service.get_task_by_id(
+        db=db,
+        task_id=task_id,
+        user_id=current_user.id
+    )
+
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Task not found"
+        )
+
+    if not task.result_json_path:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="OCR result not found"
+        )
+
+    result_json_path = Path(task.result_json_path)
+    if not result_json_path.exists():
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Result file not found"
+        )
+
+    # Find translation file
+    result_dir = result_json_path.parent
+    base_name = result_json_path.stem.replace('_result', '').replace('edit_', '')
+    translation_file = result_dir / f"{base_name}_translated_{lang}.json"
+
+    # Also try with edit_ prefix removed differently
+    if not translation_file.exists():
+        translation_file = result_dir / f"edit_translated_{lang}.json"
+
+    if not translation_file.exists():
+        # List available translations for error message
+        available = list_available_translations(result_dir)
+        if available:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail=f"Translation for language '{lang}' not found. Available translations: {', '.join(available)}"
+            )
+        else:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail=f"No translations found for this task. Please translate the document first."
+            )
+
+    # Check translation status in translation JSON
+    try:
+        with open(translation_file, 'r', encoding='utf-8') as f:
+            translation_data = json.load(f)
+
+        if not translation_data.get('translations'):
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Translation file is empty or incomplete"
+            )
+    except json.JSONDecodeError:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Invalid translation file format"
+        )
+
+    # Generate translated PDF to temp file
+    output_filename = f"{task_id}_translated_{lang}.pdf"
+
+    with tempfile.NamedTemporaryFile(suffix='.pdf', delete=False) as tmp_file:
+        output_path = Path(tmp_file.name)
+
+    try:
+        # Get source file path for images if available
+        source_file_path = None
+        if task.file_path and Path(task.file_path).exists():
+            source_file_path = Path(task.file_path)
+
+        success = pdf_generator_service.generate_translated_pdf(
+            result_json_path=result_json_path,
+            translation_json_path=translation_file,
+            output_path=output_path,
+            source_file_path=source_file_path
+        )
+
+        if not success:
+            raise HTTPException(
+                status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+                detail="Failed to generate translated PDF"
+            )
+
+        logger.info(f"Generated translated PDF for task {task_id}, lang={lang}")
+
+        return FileResponse(
+            path=str(output_path),
+            filename=output_filename,
+            media_type="application/pdf",
+            headers={
+                "Content-Disposition": f'attachment; filename="{output_filename}"'
+            }
+        )
+
+    except HTTPException:
+        # Clean up temp file on HTTP errors
+        if output_path.exists():
+            output_path.unlink()
+        raise
+    except Exception as e:
+        # Clean up temp file on unexpected errors
+        if output_path.exists():
+            output_path.unlink()
+        logger.exception(f"Failed to generate translated PDF for task {task_id}")
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"Failed to generate translated PDF: {str(e)}"
+        )