mcp: add image url support (#1)

* refactor: remove redundant validation in service and clean up code

- Remove validatePublishRequest function as gin binding already handles validation
- Remove unused errors import
- Simplify PublishContent method by relying on gin's built-in validation
- Add comprehensive image processing support with URL download capability

* 添加 MCP 说明文档，完善教程

configs/image.go

@@ -0,0 +1,14 @@
+package configs
+
+import (
+	"os"
+	"path/filepath"
+)
+
+const (
+	ImagesDir = "xiaohongshu_images"
+)
+
+func GetImagesPath() string {
+	return filepath.Join(os.TempDir(), ImagesDir)
+}