feat: 实现前端卡密管理界面

- 卡密列表展示与分页功能

- 单个/批量创建卡密

- 卡密删除与批量删除

- 卡密导出功能 (file-saver)

- 启用/禁用状态切换

- 状态判断 (有效/已使用/已失效)

- Toast 通知系统 (vue-sonner)

- 登录页面错误提示优化

- 后端登录错误消息中文化

backend/internal/models/chatgpt_account.go

@@ -0,0 +1,109 @@
+package models
+
+import (
+	"database/sql"
+	"encoding/json"
+	"time"
+)
+
+// ChatGPTAccount ChatGPT Team 账号表
+type ChatGPTAccount struct {
+	ID                  int          `json:"id"`
+	Name                string       `json:"name"`
+	AuthToken           string       `json:"-"` // Token 不输出到 JSON
+	TeamAccountID       string       `json:"team_account_id"`
+	SeatsInUse          int          `json:"seats_in_use"`
+	SeatsEntitled       int          `json:"seats_entitled"`
+	ActiveStart         sql.NullTime `json:"-"`
+	ActiveUntil         sql.NullTime `json:"-"`
+	IsActive            bool         `json:"is_active"`
+	ConsecutiveFailures int          `json:"consecutive_failures"`
+	LastCheck           sql.NullTime `json:"-"`
+	LastUsed            sql.NullTime `json:"-"`
+	CreatedAt           time.Time    `json:"created_at"`
+	UpdatedAt           sql.NullTime `json:"-"`
+}
+
+// ChatGPTAccountJSON 用于 JSON 序列化的结构
+type ChatGPTAccountJSON struct {
+	ID                  int     `json:"id"`
+	Name                string  `json:"name"`
+	TeamAccountID       string  `json:"team_account_id"`
+	SeatsInUse          int     `json:"seats_in_use"`
+	SeatsEntitled       int     `json:"seats_entitled"`
+	ActiveStart         *string `json:"active_start"`
+	ActiveUntil         *string `json:"active_until"`
+	IsActive            bool    `json:"is_active"`
+	ConsecutiveFailures int     `json:"consecutive_failures"`
+	LastCheck           *string `json:"last_check"`
+	LastUsed            *string `json:"last_used"`
+	CreatedAt           string  `json:"created_at"`
+	UpdatedAt           *string `json:"updated_at"`
+	AvailableSeats      int     `json:"available_seats"`
+	UsagePercentage     float64 `json:"usage_percentage"`
+}
+
+// MarshalJSON 自定义 JSON 序列化
+func (a ChatGPTAccount) MarshalJSON() ([]byte, error) {
+	j := ChatGPTAccountJSON{
+		ID:                  a.ID,
+		Name:                a.Name,
+		TeamAccountID:       a.TeamAccountID,
+		SeatsInUse:          a.SeatsInUse,
+		SeatsEntitled:       a.SeatsEntitled,
+		IsActive:            a.IsActive,
+		ConsecutiveFailures: a.ConsecutiveFailures,
+		CreatedAt:           a.CreatedAt.Format(time.RFC3339),
+		AvailableSeats:      a.AvailableSeats(),
+		UsagePercentage:     a.UsagePercentage(),
+	}
+
+	if a.ActiveStart.Valid {
+		s := a.ActiveStart.Time.Format(time.RFC3339)
+		j.ActiveStart = &s
+	}
+	if a.ActiveUntil.Valid {
+		s := a.ActiveUntil.Time.Format(time.RFC3339)
+		j.ActiveUntil = &s
+	}
+	if a.LastCheck.Valid {
+		s := a.LastCheck.Time.Format(time.RFC3339)
+		j.LastCheck = &s
+	}
+	if a.LastUsed.Valid {
+		s := a.LastUsed.Time.Format(time.RFC3339)
+		j.LastUsed = &s
+	}
+	if a.UpdatedAt.Valid {
+		s := a.UpdatedAt.Time.Format(time.RFC3339)
+		j.UpdatedAt = &s
+	}
+
+	return json.Marshal(j)
+}
+
+// TableName 返回表名
+func (ChatGPTAccount) TableName() string {
+	return "chatgpt_accounts"
+}
+
+// AvailableSeats 返回可用席位数量
+func (a *ChatGPTAccount) AvailableSeats() int {
+	return a.SeatsEntitled - a.SeatsInUse
+}
+
+// UsagePercentage 返回席位使用百分比
+func (a *ChatGPTAccount) UsagePercentage() float64 {
+	if a.SeatsEntitled == 0 {
+		return 0
+	}
+	return float64(a.SeatsInUse) / float64(a.SeatsEntitled) * 100
+}
+
+// IsSubscriptionValid 检查订阅是否有效
+func (a *ChatGPTAccount) IsSubscriptionValid() bool {
+	if !a.ActiveUntil.Valid {
+		return false
+	}
+	return time.Now().Before(a.ActiveUntil.Time)
+}