ui: no bulk actions on the link list #39

The link list, both in the web interface and over the API, acts on one link at a time:
enabling, disabling, deleting and editing are each a single link's own route. An operator
with many links — closing out an expired campaign's links, disabling a batch that shared one
now-dead destination, or deleting a set created for a one-off promotion — has to open, act
on, and confirm each link individually.

The list already supports search (q) and paging, so selecting a working set is not the
gap; acting on it is. A reasonable shape: checkboxes on the link list plus a bulk action bar
for enable, disable and delete, calling the existing single-link routes once per selected id
rather than a new endpoint, with the interface reporting how many succeeded and which (if
any) failed rather than treating the batch as all-or-nothing. A CLI equivalent (piping a list
of slugs or ids into an existing subcommand) would close the same gap for scripted use
without adding a batch route to the API at all.