fix: use task.files relationship to get source file path

Task model doesn't have file_path attribute directly. Use the files
relationship to access TaskFile.stored_path for source file path.

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

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

backend/app/routers/translate.py

@@ -596,8 +596,10 @@ async def download_translated_pdf(
     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)
+        if task.files and len(task.files) > 0:
+            stored_path = task.files[0].stored_path
+            if stored_path and Path(stored_path).exists():
+                source_file_path = Path(stored_path)
 
         success = pdf_generator_service.generate_translated_pdf(
             result_json_path=result_json_path,