build: 初始化构建后端并集成Gin框架，开放健康检查端口

backend/internal/config/config.go

@@ -0,0 +1,32 @@
+package config
+
+import (
+	"os"
+	"strconv"
+)
+
+type Config struct {
+	Port        int
+	Mode        string
+	MaxFileSize int64 // bytes
+}
+
+func Load() *Config {
+	cfg := &Config{
+		Port:        8080,
+		Mode:        "debug",
+		MaxFileSize: 10 << 20, // 10MB
+	}
+
+	if port := os.Getenv("GEN2D_PORT"); port != "" {
+		if p, err := strconv.Atoi(port); err == nil {
+			cfg.Port = p
+		}
+	}
+
+	if mode := os.Getenv("GEN2D_MODE"); mode != "" {
+		cfg.Mode = mode
+	}
+
+	return cfg
+}