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/settings.go

@@ -0,0 +1,54 @@
+package mdb
+
+// Setting stores non-credential runtime configuration as key/value pairs.
+// Groups: brand, rate, system, epay. Merchant credentials (pid + secret_key)
+// live in the api_keys table; notification configs live in NotificationChannel.
+// Default rows for the epay group are seeded on first startup (see
+// model/dao/mdb_table_init.go seedDefaultSettings). All other groups start
+// empty and fall back to hardcoded defaults until an admin sets them.
+// Only exception: system.jwt_secret is auto-generated on first startup.
+const (
+	SettingGroupBrand  = "brand"
+	SettingGroupRate   = "rate"
+	SettingGroupSystem = "system"
+	SettingGroupEpay   = "epay"
+)
+
+const (
+	SettingTypeString = "string"
+	SettingTypeInt    = "int"
+	SettingTypeBool   = "bool"
+	SettingTypeJSON   = "json"
+)
+
+const (
+	SettingKeyJwtSecret         = "system.jwt_secret"
+	SettingKeyOrderExpiration   = "system.order_expiration_time"
+	SettingKeyBrandSiteName     = "brand.site_name"
+	SettingKeyBrandLogoUrl      = "brand.logo_url"
+	SettingKeyBrandPageTitle    = "brand.page_title"
+	SettingKeyBrandPaySuccess   = "brand.pay_success_text"
+	SettingKeyBrandSupportUrl   = "brand.support_url"
+	SettingKeyRateForcedUsdt    = "rate.forced_usdt_rate"
+	SettingKeyRateAdjustPercent = "rate.adjust_percent"
+	SettingKeyRateOkxC2cEnabled = "rate.okx_c2c_enabled"
+	SettingKeyRateApiUrl        = "rate.api_url"
+
+	// EPAY route defaults — can be overridden via admin settings.
+	SettingKeyEpayDefaultToken    = "epay.default_token"
+	SettingKeyEpayDefaultCurrency = "epay.default_currency"
+	SettingKeyEpayDefaultNetwork  = "epay.default_network"
+)
+
+type Setting struct {
+	Group       string `gorm:"column:group;size:32;index:settings_group_index" json:"group" enums:"brand,rate,system" example:"rate"`
+	Key         string `gorm:"column:key;uniqueIndex:settings_key_uindex;size:128" json:"key" example:"rate.forced_usdt_rate"`
+	Value       string `gorm:"column:value;type:text" json:"value" example:"7.2"`
+	Type        string `gorm:"column:type;size:16;default:string" json:"type" enums:"string,int,bool,json" example:"string"`
+	Description string `gorm:"column:description;size:255" json:"description" example:"强制USDT汇率"`
+	BaseModel
+}
+
+func (s *Setting) TableName() string {
+	return "settings"
+}