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

config/config.go

@@ -0,0 +1,74 @@
+package config
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"log"
+	"os"
+	"strconv"
+
+	"github.com/joho/godotenv"
+)
+
+type Config struct {
+	AdminUsername    string
+	AdminPassword   string
+	SecretKey       string
+	JWTExpireMinutes int
+	DatabaseURL     string
+	Port            string
+}
+
+var Cfg Config
+
+func Load() {
+	// Load .env file from multiple locations (ignore errors if not found)
+	_ = godotenv.Load()
+	_ = godotenv.Load("backend/.env")
+
+	dbURL := getEnv("DATABASE_URL", "data/airwallex.db")
+	// Strip sqlite:/// prefix if present (Python SQLAlchemy format)
+	for _, prefix := range []string{"sqlite:///./", "sqlite:///", "sqlite://"} {
+		if len(dbURL) > len(prefix) && dbURL[:len(prefix)] == prefix {
+			dbURL = dbURL[len(prefix):]
+			break
+		}
+	}
+
+	Cfg = Config{
+		AdminUsername:    getEnv("ADMIN_USERNAME", "admin"),
+		AdminPassword:   getEnv("ADMIN_PASSWORD", "admin123"),
+		SecretKey:       getEnv("SECRET_KEY", generateRandomKey(32)),
+		JWTExpireMinutes: getEnvInt("JWT_EXPIRE_MINUTES", 480),
+		DatabaseURL:     dbURL,
+		Port:            getEnv("PORT", "8000"),
+	}
+}
+
+func getEnv(key, fallback string) string {
+	if val, ok := os.LookupEnv(key); ok {
+		return val
+	}
+	return fallback
+}
+
+func getEnvInt(key string, fallback int) int {
+	val, ok := os.LookupEnv(key)
+	if !ok {
+		return fallback
+	}
+	n, err := strconv.Atoi(val)
+	if err != nil {
+		log.Printf("warning: invalid integer for %s: %s, using default %d", key, val, fallback)
+		return fallback
+	}
+	return n
+}
+
+func generateRandomKey(length int) string {
+	b := make([]byte, length)
+	if _, err := rand.Read(b); err != nil {
+		log.Fatalf("failed to generate random key: %v", err)
+	}
+	return hex.EncodeToString(b)
+}