bingo: serve bingo.asxp.io too, credit asxp.io in footer

datacenterbingo.xyz is not in the .xyz zone yet, while bingo.asxp.io
already resolves. Serve the card on both (BINGO_ALT_HOST) so it is
reachable now; canonical and hreflang links keep pointing at the .xyz
domain. Footer gets a copyright line linking back to asxp.io.

Assisted-by: Claude:opus-5

views/bingo/index.erb

@@ -62,7 +62,10 @@
     </ul>
   </section>
 
-  <footer><%= t(:footer) %></footer>
+  <footer>
+    <p><%= t(:footer) %></p>
+    <p>© <%= Date.today.year %> <a href="https://asxp.io">asxp.io</a></p>
+  </footer>
 </main>
 
 <% b.cells.each do |dc| %>