feat: add admin panel with multi-chain support and notification system

- Add full admin REST API: auth, API keys, chains, chain tokens, wallets,
  orders, RPC nodes, settings, dashboard stats, notifications
- Add JWT-based admin authentication and API key auth middleware
- Add multi-chain listeners: BSC, ETH, Polygon, Plasma, EVM WebSocket
- Add RPC node health check job with automatic failover
- Add Telegram notification channel for order events
- Add installer for first-run database initialization
- Add new DB models: admin_user, api_key, chain, chain_token,
  rpc_node, settings, notification_channel
- Add order statistics aggregation (daily/monthly/by-chain)
- Serve built admin SPA from embedded www/ assets
- Add comprehensive test coverage for all new features

src/model/mdb/notification_channel.go

@@ -0,0 +1,36 @@
+package mdb
+
+const (
+	NotificationTypeTelegram = "telegram"
+	NotificationTypeWebhook  = "webhook"
+	NotificationTypeEmail    = "email"
+)
+
+const (
+	NotificationStatusEnable  = 1
+	NotificationStatusDisable = 2
+)
+
+// Notification events (JSON keys in Events column).
+const (
+	NotifyEventPaySuccess   = "pay_success"
+	NotifyEventOrderExpired = "order_expired"
+	NotifyEventDailyReport  = "daily_report"
+)
+
+// NotificationChannel represents one push target instance.
+// Config/Events are JSON strings consumed by the notify dispatcher.
+// For telegram, Config = {"bot_token":"...","chat_id":"...","proxy":"..."}.
+// For webhook, Config = {"url":"...","headers":{...},"secret":"..."}.
+type NotificationChannel struct {
+	Type    string `gorm:"column:type;size:32;index:notification_channels_type_enabled_index,priority:1" json:"type" enums:"telegram,webhook,email" example:"telegram"`
+	Name    string `gorm:"column:name;size:128" json:"name" example:"主通知群"`
+	Config  string `gorm:"column:config;type:text" json:"config" example:"{\"bot_token\":\"123:ABC\",\"chat_id\":\"456\"}"`
+	Events  string `gorm:"column:events;type:text" json:"events" example:"{\"pay_success\":true,\"order_expired\":true}"`
+	Enabled bool   `gorm:"column:enabled;default:true;index:notification_channels_type_enabled_index,priority:2" json:"enabled" example:"true"`
+	BaseModel
+}
+
+func (n *NotificationChannel) TableName() string {
+	return "notification_channels"
+}