feat(posts): add preview URL fields for post images

Add PreviewURL and PreviewURLLarge fields to PostImage model and responses. Image data now supports format "url|preview_url|preview_url_large" for storing multiple image variants. Also change comment cursor pagination to sort by created_at ASC (oldest first) to display earliest comments as first floor, and make AdminReviewVerificationRequest.Approve field optional.

internal/repository/post_repo.go

@@ -148,11 +148,22 @@ func (r *postRepository) UpdateWithImages(post *model.Post, images *[]string) er
 			return err
 		}
 
-		for i, url := range *images {
+		for i, imageData := range *images {
+			parts := strings.Split(imageData, "|")
+			url := parts[0]
+			var previewURL, previewURLLarge string
+			if len(parts) >= 2 && parts[1] != "" {
+				previewURL = parts[1]
+			}
+			if len(parts) >= 3 && parts[2] != "" {
+				previewURLLarge = parts[2]
+			}
 			image := &model.PostImage{
-				PostID:    post.ID,
-				URL:       url,
-				SortOrder: i,
+				PostID:          post.ID,
+				URL:             url,
+				PreviewURL:      previewURL,
+				PreviewURLLarge: previewURLLarge,
+				SortOrder:       i,
 			}
 			if err := tx.Create(image).Error; err != nil {
 				return err
@@ -695,11 +706,23 @@ func (r *postRepository) createWithTx(tx *gorm.DB, post *model.Post, images []st
 	}
 
 	// 创建图片记录
-	for i, url := range images {
+	// 支持格式：["url", "url|preview_url|preview_url_large", ...]
+	for i, imageData := range images {
+		parts := strings.Split(imageData, "|")
+		url := parts[0]
+		var previewURL, previewURLLarge string
+		if len(parts) >= 2 && parts[1] != "" {
+			previewURL = parts[1]
+		}
+		if len(parts) >= 3 && parts[2] != "" {
+			previewURLLarge = parts[2]
+		}
 		image := &model.PostImage{
-			PostID:    post.ID,
-			URL:       url,
-			SortOrder: i,
+			PostID:          post.ID,
+			URL:             url,
+			PreviewURL:      previewURL,
+			PreviewURLLarge: previewURLLarge,
+			SortOrder:       i,
 		}
 		if err := tx.Create(image).Error; err != nil {
 			return err