feat(flask): OHLCV 端点自动附加 ETF 净值和溢价率

flask_server.py:
- 当 asset_type 为 china_etf 时，自动调用 fetch_etf_with_nav
- 响应中添加 nav、premium_series、latest_premium、premium_stats

flask_api_source.py:
- 解析 ETF 数据中的净值和溢价率信息
- 将 nav_df、premium_series、premium_stats 存入 DataFrame.attrs

datasource/flask_api_source.py

@@ -142,6 +142,30 @@ class FlaskAPIDataSource:
                 if 'info' in data:
                     df.attrs['info'] = data['info']
                 
+                # ETF 数据自动附加净值和溢价率信息
+                if data.get('asset_type') == 'china_etf':
+                    # 净值数据
+                    nav_section = data.get('nav', {})
+                    if nav_section.get('data'):
+                        nav_df = pd.DataFrame(nav_section['data'])
+                        if 'date' in nav_df.columns:
+                            nav_df['date'] = pd.to_datetime(nav_df['date'])
+                            nav_df = nav_df.set_index('date')
+                        df.attrs['nav'] = nav_df
+                    
+                    # 溢价率序列
+                    if 'premium_series' in data:
+                        df.attrs['premium_series'] = data['premium_series']
+                    
+                    # 最新溢价率
+                    if 'latest_premium' in data:
+                        df.attrs['latest_premium'] = data['latest_premium']
+                        df.attrs['premium_date'] = data.get('premium_date')
+                    
+                    # 溢价率统计
+                    if 'premium_stats' in data:
+                        df.attrs['premium_stats'] = data['premium_stats']
+                
                 print(f"✓ {code}: {len(df)} 条数据 ({start_date} ~ {end_date})")
                 return df