fix(auth): encode snowflake user ids as strings in session and /user-info

Registered users get snowflake ids above JS MAX_SAFE_INTEGER.
/user-info emitted them as JSON numbers and login stored uint64 in
the session. Both now use decimal strings. Tests cover admin vs
non-admin cookie access to /user/self, /user-info, and /upload/my.

backend/plugins/domain/auth/models.go

@@ -146,7 +146,7 @@ type CallbackRequest struct {
 
 // BasicUserInfo 用户基本信息结构体
 type BasicUserInfo struct {
-	ID                 uint64 `json:"id"`
+	ID                 uint64 `json:"id,string"`
 	Username           string `json:"username"`
 	Nickname           string `json:"nickname"`
 	Email              string `json:"email"`