Set and store report URIs (#10303)

Fixes #10271

app/lib/activitypub/activity/flag.rb

@@ -14,7 +14,8 @@ class ActivityPub::Activity::Flag < ActivityPub::Activity
         @account,
         target_account,
         status_ids: target_statuses.nil? ? [] : target_statuses.map(&:id),
-        comment: @json['content'] || ''
+        comment: @json['content'] || '',
+        uri: report_uri
       )
     end
   end
@@ -28,4 +29,8 @@ class ActivityPub::Activity::Flag < ActivityPub::Activity
   def object_uris
     @object_uris ||= Array(@object.is_a?(Array) ? @object.map { |item| value_or_id(item) } : value_or_id(@object))
   end
+
+  def report_uri
+    @json['id'] unless @json['id'].nil? || invalid_origin?(@json['id'])
+  end
 end