backend: adapt to /media/tags/:media_id API change

templates/media.html.ep

@@ -14,11 +14,11 @@
   <dt>Content-Type</dt>
   <dd><%= $media->{content_type} %></dd>
   <dt>Tags</dt>
-% if ($media->{tags}->@* == 0) {
+% if ($tags->@* == 0) {
   <dd>None yet.</dd>
 % } else {
   <dd>
-%   for my $tag ($media->{tags}->@*) {
+%   for my $tag ($tags->@*) {
     <%= link_to $tag => url_for("tag_show", tag_id_or_name => $tag) %>
 %   }
   </dd>