Force .pdf extension on uploaded files regardless of user input

The file content is already validated as PDF, so the user-supplied
extension should not be trusted or written to disk.

document-parser/services/document_service.py

@@ -32,7 +32,7 @@ async def upload(filename: str, content_type: str, file_content: bytes) -> Docum
 
     os.makedirs(UPLOAD_DIR, exist_ok=True)
 
-    ext = os.path.splitext(filename)[1] or ".pdf"
+    ext = ".pdf"  # Content already validated as PDF
     safe_name = f"{uuid.uuid4()}{ext}"
     file_path = os.path.join(UPLOAD_DIR, safe_name)