build: 初始化构建后端并集成Gin框架，开放健康检查端口

backend/internal/handler/health.go

@@ -0,0 +1,11 @@
+package handler
+
+import (
+	"gen2d/internal/model"
+
+	"github.com/gin-gonic/gin"
+)
+
+func Health(c *gin.Context) {
+	c.JSON(200, model.OK(gin.H{"status": "healthy"}))
+}