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/model/account.go

@@ -4,6 +4,7 @@ import (
 	"database/sql/driver"
 	"encoding/json"
 	"errors"
+	"strconv"
 	"time"
 
 	"gorm.io/gorm"
@@ -128,8 +129,37 @@ func (a *Account) GetCredential(key string) string {
 		return ""
 	}
 	if v, ok := a.Credentials[key]; ok {
-		if s, ok := v.(string); ok {
-			return s
+		switch vv := v.(type) {
+		case string:
+			return vv
+		case json.Number:
+			return vv.String()
+		case float64:
+			// JSON numbers decode to float64; keep integer formatting for integer-like values.
+			i := int64(vv)
+			if vv == float64(i) {
+				return strconv.FormatInt(i, 10)
+			}
+			return strconv.FormatFloat(vv, 'f', -1, 64)
+		case float32:
+			f := float64(vv)
+			i := int64(f)
+			if f == float64(i) {
+				return strconv.FormatInt(i, 10)
+			}
+			return strconv.FormatFloat(f, 'f', -1, 64)
+		case int:
+			return strconv.FormatInt(int64(vv), 10)
+		case int64:
+			return strconv.FormatInt(vv, 10)
+		case int32:
+			return strconv.FormatInt(int64(vv), 10)
+		case uint:
+			return strconv.FormatUint(uint64(vv), 10)
+		case uint64:
+			return strconv.FormatUint(vv, 10)
+		case uint32:
+			return strconv.FormatUint(uint64(vv), 10)
 		}
 	}
 	return ""
@@ -291,6 +321,11 @@ func (a *Account) IsAnthropic() bool {
 	return a.Platform == PlatformAnthropic
 }
 
+// IsGemini 检查是否为 Gemini 平台账号
+func (a *Account) IsGemini() bool {
+	return a.Platform == PlatformGemini
+}
+
 // IsOpenAIOAuth 检查是否为 OpenAI OAuth 类型账号
 func (a *Account) IsOpenAIOAuth() bool {
 	return a.IsOpenAI() && a.Type == AccountTypeOAuth