fix(datasource): 修复溢价率计算重复日期导致的 reindex 失败

问题：长时间范围 ETF 数据获取时，出现 'cannot reindex on an axis with duplicate labels' 错误

修复：
- 在 _calculate_premium_series 中先检测并去除重复日期
- price_df 和 nav_df 的索引都使用 duplicated(keep='last') 去重
- 确保 reindex 操作正常执行

datasource/universal_fetcher.py

@@ -247,7 +247,17 @@ class UniversalDataFetcher:
         """
         # 对齐日期：净值用ffill填充（因为T+1公布）
         # 价格日期可能比净值日期多一天
-        aligned_nav = nav_df['nav'].reindex(price_df.index, method='ffill')
+        # 先去除重复日期
+        price_index = price_df.index
+        if price_index.has_duplicates:
+            price_df = price_df[~price_df.index.duplicated(keep='last')]
+            price_index = price_df.index
+        
+        nav_index = nav_df.index
+        if nav_index.has_duplicates:
+            nav_df = nav_df[~nav_df.index.duplicated(keep='last')]
+        
+        aligned_nav = nav_df['nav'].reindex(price_index, method='ffill')
         
         # 计算溢价率
         close_prices = price_df['close']