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/services/pdf_generator_service.py

@@ -3601,6 +3601,100 @@ class PDFGeneratorService:
         except Exception as e:
             logger.error(f"Failed to draw image element {element.element_id}: {e}")
 
+    def generate_translated_pdf(
+        self,
+        result_json_path: Path,
+        translation_json_path: Path,
+        output_path: Path,
+        source_file_path: Optional[Path] = None
+    ) -> bool:
+        """
+        Generate layout-preserving PDF with translated content.
+
+        This method loads the original result JSON and translation JSON,
+        merges them to replace original content with translations, and
+        generates a PDF with the translated content at original positions.
+
+        Args:
+            result_json_path: Path to original result JSON file (UnifiedDocument format)
+            translation_json_path: Path to translation JSON file
+            output_path: Path to save generated translated PDF
+            source_file_path: Optional path to original source file
+
+        Returns:
+            True if successful, False otherwise
+        """
+        import tempfile
+
+        try:
+            # Import apply_translations from translation service
+            from app.services.translation_service import apply_translations
+
+            # Load original result JSON
+            logger.info(f"Loading result JSON: {result_json_path}")
+            with open(result_json_path, 'r', encoding='utf-8') as f:
+                result_json = json.load(f)
+
+            # Load translation JSON
+            logger.info(f"Loading translation JSON: {translation_json_path}")
+            with open(translation_json_path, 'r', encoding='utf-8') as f:
+                translation_json = json.load(f)
+
+            # Extract translations dict from translation JSON
+            translations = translation_json.get('translations', {})
+            if not translations:
+                logger.warning("No translations found in translation JSON")
+                # Still generate PDF with original content as fallback
+                return self.generate_layout_pdf(
+                    json_path=result_json_path,
+                    output_path=output_path,
+                    source_file_path=source_file_path
+                )
+
+            # Apply translations to result JSON
+            translated_doc = apply_translations(result_json, translations)
+
+            target_lang = translation_json.get('target_lang', 'unknown')
+            logger.info(
+                f"Generating translated PDF: {len(translations)} translations applied, "
+                f"target_lang={target_lang}"
+            )
+
+            # Write translated JSON to a temporary file and use existing generate_layout_pdf
+            with tempfile.NamedTemporaryFile(
+                mode='w',
+                suffix='_translated.json',
+                delete=False,
+                encoding='utf-8'
+            ) as tmp_file:
+                json.dump(translated_doc, tmp_file, ensure_ascii=False, indent=2)
+                tmp_path = Path(tmp_file.name)
+
+            try:
+                # Use existing PDF generation with translated content
+                success = self.generate_layout_pdf(
+                    json_path=tmp_path,
+                    output_path=output_path,
+                    source_file_path=source_file_path
+                )
+                return success
+            finally:
+                # Clean up temporary file
+                if tmp_path.exists():
+                    tmp_path.unlink()
+
+        except FileNotFoundError as e:
+            logger.error(f"File not found: {e}")
+            return False
+        except json.JSONDecodeError as e:
+            logger.error(f"Invalid JSON: {e}")
+            return False
+        except Exception as e:
+            logger.error(f"Failed to generate translated PDF: {e}")
+            import traceback
+            traceback.print_exc()
+            return False
+
 
 # Singleton instance
 pdf_generator_service = PDFGeneratorService()