fix(api): standardize UTC datetime serialization to use 'Z' instead of '+00:00' to support some service

e.g. https://github.com/yaowan233/nonebot-plugin-osubot

app/models/model.py

@@ -13,7 +13,7 @@ class UTCBaseModel(BaseModel):
         if isinstance(v, datetime):
             if v.tzinfo is None:
                 v = v.replace(tzinfo=UTC)
-            return v.astimezone(UTC).isoformat()
+            return v.astimezone(UTC).isoformat().replace("+00:00", "Z")
         return v