feat(backend): 完善 Gemini OAuth Token 处理

- 修复 account_handler 中 token 字段类型转换（int64 转 string）
- 增强 Account.GetCredential 支持多种数值类型（float64, int, json.Number 等）
- 添加 Account.IsGemini() 方法用于平台判断
- 优化 refresh_token 和 scope 的空值处理

backend/internal/handler/admin/account_handler.go

@@ -2,6 +2,7 @@ package admin
 
 import (
 	"strconv"
+	"strings"
 
 	"github.com/Wei-Shaw/sub2api/internal/model"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/claude"
@@ -375,18 +376,22 @@ func (h *AccountHandler) Refresh(c *gin.Context) {
 			newCredentials[k] = v
 		}
 
-		// Update token-related fields
-		newCredentials["access_token"] = tokenInfo.AccessToken
-		newCredentials["token_type"] = tokenInfo.TokenType
-		newCredentials["expires_in"] = tokenInfo.ExpiresIn
-		newCredentials["expires_at"] = tokenInfo.ExpiresAt
-		newCredentials["refresh_token"] = tokenInfo.RefreshToken
-		newCredentials["scope"] = tokenInfo.Scope
-	}
+			// Update token-related fields
+			newCredentials["access_token"] = tokenInfo.AccessToken
+			newCredentials["token_type"] = tokenInfo.TokenType
+			newCredentials["expires_in"] = strconv.FormatInt(tokenInfo.ExpiresIn, 10)
+			newCredentials["expires_at"] = strconv.FormatInt(tokenInfo.ExpiresAt, 10)
+			if strings.TrimSpace(tokenInfo.RefreshToken) != "" {
+				newCredentials["refresh_token"] = tokenInfo.RefreshToken
+			}
+			if strings.TrimSpace(tokenInfo.Scope) != "" {
+				newCredentials["scope"] = tokenInfo.Scope
+			}
+		}
 
-	updatedAccount, err := h.adminService.UpdateAccount(c.Request.Context(), accountID, &service.UpdateAccountInput{
-		Credentials: newCredentials,
-	})
+		updatedAccount, err := h.adminService.UpdateAccount(c.Request.Context(), accountID, &service.UpdateAccountInput{
+			Credentials: newCredentials,
+		})
 	if err != nil {
 		response.ErrorFrom(c, err)
 		return