media assets: allow assets to be regenerated.

Add a `MediaAsset#regenerate!` method that regenerates everything about
the asset, including the metadata, thumbnails, IQDB, cached Cloudflare
URLs, and AI tags.

Fixes it so that a) it's possible to regenerate media assets that aren't
attached to posts and b) regenerating a post regenerates everything. Before
it didn't regenerate the metadata, AI tags, or all of the cached URLs.

app/models/post.rb

@@ -1454,38 +1454,11 @@ class Post < ApplicationRecord
 
         ModAction.log("regenerated IQDB for post ##{id}", :post_regenerate_iqdb, subject: self, user: user)
       else
-        media_file = media_asset.variant(:original).open_file
-        media_asset.distribute_files!(media_file)
-
-        update!(
-          image_width: media_file.width,
-          image_height: media_file.height,
-          file_size: media_file.file_size,
-          file_ext: media_file.file_ext
-        )
-
-        media_asset.update!(
-          image_width: media_file.width,
-          image_height: media_file.height,
-          file_size: media_file.file_size,
-          file_ext: media_file.file_ext
-        )
-
-        purge_cached_urls!
-        update_iqdb
+        media_asset.regenerate!
 
         ModAction.log("regenerated image samples for post ##{id}", :post_regenerate, subject: self, user: user)
       end
     end
-
-    def purge_cached_urls!
-      urls = [
-        preview_file_url, large_file_url, file_url,
-        tagged_file_url(tagged_filenames: true), tagged_large_file_url(tagged_filenames: true),
-      ]
-
-      CloudflareService.new.purge_cache(urls)
-    end
   end
 
   concerning :IqdbMethods do