Fix last_fetched_at not being set on polls (mastodon#10170)

Author: Gargron

app/services/activitypub/fetch_remote_poll_service.rb

@@ -33,6 +33,7 @@ def call(poll, on_behalf_of = nil)
     poll.votes.delete_all if latest_options != poll.options
 
     poll.update!(
+      last_fetched_at: Time.now.utc,
       expires_at: expires_at,
       options: latest_options,
       cached_tallies: items.map { |item| item.dig('replies', 'totalItems') || 0 }