fix(opnform): address review feedback on vars header and meta boilerplate

* vars/main.yml: header was 'vars file for homarr' (copy-paste from the
  homarr role). Fixed to 'vars file for opnform'. File body is empty.
* meta/main.yml: replace ansible-galaxy init boilerplate with real
  metadata — author, description, license (MIT-0), min_ansible_version
  set to '2.15' as a string (galaxy schema requires str), galaxy_tags
  for discovery, and an empty dependencies list.

The third inline finding (dead roles/opnform/templates/compose.yml.j2)
is resolved by dropping the WIP commit a6f301e during the rebase rather
than removing it in a separate commit — the file no longer exists in
the rebased history.

roles/opnform/vars/main.yml

@@ -1,3 +1,3 @@
 #SPDX-License-Identifier: MIT-0
 ---
-# vars file for homarr
+# vars file for opnform