feat: Go 重写后端，替换 Python FastAPI

用 Go (Gin + GORM + SQLite) 重写整个后端：
- 单二进制部署，不依赖 Python/pip/SDK
- net/http 原生客户端，无 Cloudflare TLS 指纹问题
- 多阶段 Dockerfile：Node 构建前端 + Go 构建后端 + Alpine 运行
- 内存占用从 ~95MB 降至 ~3MB
- 完整保留所有 API 路由、JWT 认证、API Key 权限、审计日志

models/init.go

@@ -0,0 +1,40 @@
+package models
+
+import (
+	"log"
+	"os"
+
+	"airwallex-admin/config"
+
+	"golang.org/x/crypto/bcrypt"
+	"gorm.io/gorm"
+)
+
+func InitializeDefaults(db *gorm.DB) {
+	// Set admin password hash if not already set
+	if GetSetting(db, "admin_password_hash") == "" {
+		hash, err := bcrypt.GenerateFromPassword([]byte(config.Cfg.AdminPassword), bcrypt.DefaultCost)
+		if err != nil {
+			log.Fatalf("failed to hash admin password: %v", err)
+		}
+		SetSetting(db, "admin_password_hash", string(hash), false)
+	}
+
+	// Set default daily card limit
+	if GetSetting(db, "daily_card_limit") == "" {
+		SetSetting(db, "daily_card_limit", "100", false)
+	}
+
+	// Store Airwallex credentials from environment if not already set
+	envSettings := map[string]string{
+		"airwallex_client_id": os.Getenv("AIRWALLEX_CLIENT_ID"),
+		"airwallex_api_key":   os.Getenv("AIRWALLEX_API_KEY"),
+		"airwallex_base_url":  os.Getenv("AIRWALLEX_BASE_URL"),
+	}
+
+	for key, value := range envSettings {
+		if value != "" && GetSetting(db, key) == "" {
+			SetSetting(db, key, value, key != "airwallex_base_url")
+		}
+	}
+}