Add preview of followers removed in domain block modal in web UI (#32032)

app/controllers/api/v1/domain_blocks/previews_controller.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class Api::V1::DomainBlocks::PreviewsController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :follow, :write, :'write:blocks' }
+  before_action :require_user!
+  before_action :set_domain
+  before_action :set_domain_block_preview
+
+  def show
+    render json: @domain_block_preview, serializer: REST::DomainBlockPreviewSerializer
+  end
+
+  private
+
+  def set_domain
+    @domain = TagManager.instance.normalize_domain(params[:domain])
+  end
+
+  def set_domain_block_preview
+    @domain_block_preview = with_read_replica do
+      DomainBlockPreviewPresenter.new(
+        following_count: current_account.following.where(domain: @domain).count,
+        followers_count: current_account.followers.where(domain: @domain).count
+      )
+    end
+  end
+end