merge: 合并 upstream/main 解决 PR #37 冲突

- 删除 backend/internal/model/account.go 符合重构方向
- 合并最新的项目结构重构
- 包含 SSE 格式解析修复
- 更新依赖和配置文件

backend/internal/server/middleware/middleware.go

@@ -8,6 +8,8 @@ type ContextKey string
 const (
 	// ContextKeyUser 用户上下文键
 	ContextKeyUser ContextKey = "user"
+	// ContextKeyUserRole 当前用户角色（string）
+	ContextKeyUserRole ContextKey = "user_role"
 	// ContextKeyApiKey API密钥上下文键
 	ContextKeyApiKey ContextKey = "api_key"
 	// ContextKeySubscription 订阅上下文键

backend/internal/service/account_group.go

+package service
+
+import "time"
+
+type AccountGroup struct {
+	AccountID int64
+	GroupID   int64
+	Priority  int
+	CreatedAt time.Time
+
+	Account *Account
+	Group   *Group
+}