Fix preview card image not being re-fetched even if link is re-posted (#11981)

Fix #11956

app/models/preview_card.rb

@@ -47,6 +47,10 @@ class PreviewCard < ApplicationRecord
 
   before_save :extract_dimensions, if: :link?
 
+  def missing_image?
+    width.present? && height.present? && image_file_name.blank?
+  end
+
   def save_with_optional_image!
     save!
   rescue ActiveRecord::RecordInvalid

app/services/fetch_link_card_service.rb

@@ -22,7 +22,7 @@ class FetchLinkCardService < BaseService
     RedisLock.acquire(lock_options) do |lock|
       if lock.acquired?
         @card = PreviewCard.find_by(url: @url)
-        process_url if @card.nil? || @card.updated_at <= 2.weeks.ago
+        process_url if @card.nil? || @card.updated_at <= 2.weeks.ago || @card.missing_image?
       else
         raise Mastodon::RaceConditionError
       end