refactor(utils/datetime): rewrite utc datetime formatting logic

明确 naive 时区 datetime 的处理逻辑，补充和数据库存储约定的注释，将原逻辑拆分为显式的 naive/aware 分支处理，保持原有功能不变

backend/package/yuxi/utils/datetime_utils.py

@@ -120,10 +120,15 @@ def format_utc_datetime(value: dt.datetime | None) -> str | None:
     Format a datetime to UTC ISO 8601 string, handling naive datetimes.
 
     Returns None for None input.
-    Naive datetimes are assumed to be in UTC.
+    Naive datetimes are assumed to be in UTC (与 DB 存储约定一致：所有 DateTime
+    列以 UTC naive 存储)。aware datetimes 统一转换为 UTC。
     """
     if value is None:
         return None
+    if value.tzinfo is None:
+        # naive datetime 视为 UTC，直接附加 Z 后缀
+        return value.isoformat() + "Z"
+    # aware datetime 转换为 UTC
     return utc_isoformat(value)