修复了一个缩进错误

api/services/poster.py

@@ -672,10 +672,10 @@ class PosterService:
             
             # 生成JSON的base64编码
             json_base64 = None
-                try:
+            try:
                 json_string = json.dumps(fabric_json, ensure_ascii=False)
                 json_base64 = base64.b64encode(json_string.encode('utf-8')).decode('utf-8')
-                except Exception as e:
+            except Exception as e:
                 logger.warning(f"生成JSON base64编码失败: {e}")
             
             logger.info(f"Fabric.js JSON文件生成成功: {json_path} ({file_size/1024:.1f}KB)")