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/response/base.go

@@ -0,0 +1,11 @@
+package response
+
+// ApiResponse is the standard JSON envelope used by all API endpoints.
+// Used for swagger documentation only — the actual response is built
+// by util/http.Resp.
+type ApiResponse struct {
+	StatusCode int         `json:"status_code" example:"200"`
+	Message    string      `json:"message" example:"success"`
+	Data       interface{} `json:"data"`
+	RequestID  string      `json:"request_id" example:"req-123456"`
+}