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/chain.go

@@ -0,0 +1,19 @@
+package mdb
+
+// Chain represents an enabled blockchain network.
+// Scanner tasks read this table on each polling tick to decide whether to
+// process the network. When Enabled=false the scanner skips the chain
+// entirely without restarting.
+type Chain struct {
+	Network          string `gorm:"column:network;uniqueIndex:chains_network_uindex;size:32" json:"network" example:"tron"`
+	DisplayName      string `gorm:"column:display_name;size:64" json:"display_name" example:"Tron"`
+	Enabled          bool   `gorm:"column:enabled;default:true" json:"enabled" example:"true"`
+	MinConfirmations int    `gorm:"column:min_confirmations;default:1" json:"min_confirmations" example:"20"`
+	ScanIntervalSec  int    `gorm:"column:scan_interval_sec;default:5" json:"scan_interval_sec" example:"5"`
+	Extra            string `gorm:"column:extra;type:text" json:"extra" example:"{}"`
+	BaseModel
+}
+
+func (c *Chain) TableName() string {
+	return "chains"
+}