fix(flask_api_source): 修复 fetch_etf_nav zstd 解压与溢价率解析

- 处理 zstd 响应的 JSON 解析问题
- 正确解析 Flask server 返回的净值数据结构
- 添加 premium_series、latest_premium、premium_stats 到 DataFrame attrs

datasource/flask_api_source.py

@@ -231,13 +231,19 @@ class FlaskAPIDataSource:
             if response.status_code != 200:
                 return None
             
-            data = response.json()
+            # 处理 zstd 响应
+            try:
+                data = response.json()
+            except (json.JSONDecodeError, requests.exceptions.JSONDecodeError):
+                data = json.loads(response.text)
             
-            if 'error' in data or 'nav' not in data:
+            if 'error' in data:
                 return None
             
-            nav_data = data.get('nav', {})
-            records = nav_data.get('data', [])
+            # 解析净值数据
+            # Flask server 返回格式: {'nav': {'data': [...], 'count': N}, 'premium_series': [...]}
+            nav_section = data.get('nav', {})
+            records = nav_section.get('data', [])
             
             if not records:
                 return None
@@ -247,6 +253,14 @@ class FlaskAPIDataSource:
                 df['date'] = pd.to_datetime(df['date'])
                 df = df.set_index('date')
             
+            # 添加溢价率信息（如果有）
+            if 'premium_series' in data:
+                df.attrs['premium_series'] = data['premium_series']
+            if 'latest_premium' in data:
+                df.attrs['latest_premium'] = data['latest_premium']
+            if 'premium_stats' in data:
+                df.attrs['premium_stats'] = data['premium_stats']
+            
             return df
             
         except Exception as e: