fix(backend): 适配重构后的架构修复 Gemini OAuth 集成

## 主要修改

1. **移除 model 包引用**
   - 删除所有 `internal/model` 包的 import
   - 使用 service 包中的类型定义（Account, Platform常量等）

2. **修复类型转换**
   - JSONB → map[string]any
   - 添加 mergeJSONB 辅助函数
   - 添加 Account.IsGemini() 方法

3. **更新中间件调用**
   - GetUserFromContext → GetAuthSubjectFromContext
   - 适配新的并发控制签名（传递 ID 和 Concurrency 而不是完整对象）

4. **修复 handler 层**
   - 更新 gemini_v1beta_handler.go
   - 修正 billing 检查和 usage 记录

## 影响范围
- backend/internal/service/gemini_*.go
- backend/internal/service/account_test_service.go
- backend/internal/service/crs_sync_service.go
- backend/internal/handler/gemini_v1beta_handler.go
- backend/internal/handler/gateway_handler.go
- backend/internal/handler/admin/account_handler.go

backend/internal/service/gemini_token_refresher.go

@@ -5,7 +5,6 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/Wei-Shaw/sub2api/internal/model"
 )
 
 type GeminiTokenRefresher struct {
@@ -16,11 +15,11 @@ func NewGeminiTokenRefresher(geminiOAuthService *GeminiOAuthService) *GeminiToke
 	return &GeminiTokenRefresher{geminiOAuthService: geminiOAuthService}
 }
 
-func (r *GeminiTokenRefresher) CanRefresh(account *model.Account) bool {
-	return account.Platform == model.PlatformGemini && account.Type == model.AccountTypeOAuth
+func (r *GeminiTokenRefresher) CanRefresh(account *Account) bool {
+	return account.Platform == PlatformGemini && account.Type == AccountTypeOAuth
 }
 
-func (r *GeminiTokenRefresher) NeedsRefresh(account *model.Account, refreshWindow time.Duration) bool {
+func (r *GeminiTokenRefresher) NeedsRefresh(account *Account, refreshWindow time.Duration) bool {
 	if !r.CanRefresh(account) {
 		return false
 	}
@@ -36,7 +35,7 @@ func (r *GeminiTokenRefresher) NeedsRefresh(account *model.Account, refreshWindo
 	return time.Until(expiryTime) < refreshWindow
 }
 
-func (r *GeminiTokenRefresher) Refresh(ctx context.Context, account *model.Account) (map[string]any, error) {
+func (r *GeminiTokenRefresher) Refresh(ctx context.Context, account *Account) (map[string]any, error) {
 	tokenInfo, err := r.geminiOAuthService.RefreshAccountToken(ctx, account)
 	if err != nil {
 		return nil, err