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

pkg/downloader/images.go

@@ -0,0 +1,148 @@
+package downloader
+
+import (
+	"crypto/sha256"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/h2non/filetype"
+	"github.com/pkg/errors"
+)
+
+// ImageDownloader 图片下载器
+type ImageDownloader struct {
+	savePath   string
+	httpClient *http.Client
+}
+
+// NewImageDownloader 创建图片下载器
+func NewImageDownloader(savePath string) *ImageDownloader {
+	// 确保保存目录存在
+	if err := os.MkdirAll(savePath, 0755); err != nil {
+		panic(fmt.Sprintf("failed to create save path: %v", err))
+	}
+
+	return &ImageDownloader{
+		savePath: savePath,
+		httpClient: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+// DownloadImage 下载图片
+// 返回本地文件路径
+func (d *ImageDownloader) DownloadImage(imageURL string) (string, error) {
+	// 验证URL格式
+	if !d.isValidImageURL(imageURL) {
+		return "", errors.New("invalid image URL format")
+	}
+
+	// 下载图片数据
+	resp, err := d.httpClient.Get(imageURL)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to download image")
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("download failed with status: %d", resp.StatusCode)
+	}
+
+	// 读取图片数据
+	imageData, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to read image data")
+	}
+
+	// 检测图片格式
+	kind, err := filetype.Match(imageData)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to detect file type")
+	}
+
+	if !filetype.IsImage(imageData) {
+		return "", errors.New("downloaded file is not a valid image")
+	}
+
+	// 生成唯一文件名
+	fileName := d.generateFileName(imageURL, kind.Extension)
+	filePath := filepath.Join(d.savePath, fileName)
+
+	// 如果文件已存在，直接返回路径
+	if _, err := os.Stat(filePath); err == nil {
+		return filePath, nil
+	}
+
+	// 保存到文件
+	if err := os.WriteFile(filePath, imageData, 0644); err != nil {
+		return "", errors.Wrap(err, "failed to save image")
+	}
+
+	return filePath, nil
+}
+
+// DownloadImages 批量下载图片
+func (d *ImageDownloader) DownloadImages(imageURLs []string) ([]string, error) {
+	var localPaths []string
+	var errs []error
+
+	for _, imageURL := range imageURLs {
+		localPath, err := d.DownloadImage(imageURL)
+		if err != nil {
+			errs = append(errs, fmt.Errorf("failed to download %s: %w", imageURL, err))
+			continue
+		}
+		localPaths = append(localPaths, localPath)
+	}
+
+	if len(errs) > 0 {
+		return localPaths, fmt.Errorf("download errors occurred: %v", errs)
+	}
+
+	return localPaths, nil
+}
+
+// isValidImageURL 检查是否为有效的图片URL
+func (d *ImageDownloader) isValidImageURL(rawURL string) bool {
+	// 检查是否以http/https开头
+	if !strings.HasPrefix(strings.ToLower(rawURL), "http://") &&
+		!strings.HasPrefix(strings.ToLower(rawURL), "https://") {
+		return false
+	}
+
+	// 检查URL格式
+	parsedURL, err := url.Parse(rawURL)
+	if err != nil {
+		return false
+	}
+
+	return parsedURL.Scheme != "" && parsedURL.Host != ""
+}
+
+// generateFileName 生成唯一的文件名
+func (d *ImageDownloader) generateFileName(imageURL, extension string) string {
+	// 使用URL的SHA256哈希作为文件名，确保唯一性
+	hash := sha256.Sum256([]byte(imageURL))
+	hashStr := fmt.Sprintf("%x", hash)
+
+	// 取前16位哈希值作为文件名
+	shortHash := hashStr[:16]
+
+	// 添加时间戳确保更好的唯一性
+	timestamp := time.Now().Unix()
+
+	return fmt.Sprintf("img_%s_%d.%s", shortHash, timestamp, extension)
+}
+
+// IsImageURL 判断字符串是否为图片URL
+func IsImageURL(path string) bool {
+	return strings.HasPrefix(strings.ToLower(path), "http://") ||
+		strings.HasPrefix(strings.ToLower(path), "https://")
+}