feat: 初始化 ChatGPT Team 管理后端项目

- 添加用户认证模块 (JWT + 密码管理)
- 添加 ChatGPT 账户管理功能
- 添加卡密管理功能 (创建、批量生成、查询)
- 添加邀请功能
- 配置数据库迁移和路由系统

internal/models/invitation.go

@@ -0,0 +1,43 @@
+package models
+
+import (
+	"database/sql"
+	"time"
+)
+
+// InvitationStatus 邀请状态枚举
+type InvitationStatus string
+
+const (
+	StatusPending  InvitationStatus = "pending"
+	StatusSent     InvitationStatus = "sent"
+	StatusAccepted InvitationStatus = "accepted"
+	StatusFailed   InvitationStatus = "failed"
+	StatusExpired  InvitationStatus = "expired"
+)
+
+// Invitation 邀请记录表
+type Invitation struct {
+	ID           int              `json:"id"`
+	CardKeyID    sql.NullInt64    `json:"card_key_id"`
+	AccountID    int              `json:"account_id"`
+	InvitedEmail string           `json:"invited_email"`
+	Status       InvitationStatus `json:"status"`
+	ErrorMessage sql.NullString   `json:"error_message"`
+	ExpiresAt    sql.NullTime     `json:"expires_at"`
+	CreatedAt    time.Time        `json:"created_at"`
+	UpdatedAt    sql.NullTime     `json:"updated_at"`
+}
+
+// TableName 返回表名
+func (Invitation) TableName() string {
+	return "invitations"
+}
+
+// IsExpired 检查邀请是否过期
+func (i *Invitation) IsExpired() bool {
+	if !i.ExpiresAt.Valid {
+		return false
+	}
+	return time.Now().After(i.ExpiresAt.Time)
+}