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/dto/dto.go

@@ -1404,7 +1404,7 @@ type AdminVerificationDetailResponse struct {
 
 // AdminReviewVerificationRequest 管理端审核认证请求
 type AdminReviewVerificationRequest struct {
-	Approve      bool   `json:"approve" binding:"required"`
+	Approve      bool   `json:"approve"`
 	RejectReason string `json:"reject_reason" binding:"omitempty,max=500"`
 }