The Commet role was the only playbook-installed web client without a
`_scheme` variable to go with its `_hostname` and `_path_prefix`, so
anything wishing to build Commet's public URL had to reach for
`matrix_playbook_ssl_enabled` itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Replaces `community.docker.docker_image` with the modern
`docker_image_pull` module. Drops the `ansible_version` compatibility
ladder and the now-redundant `_container_image_force_pull` variable
(the new pull module handles registry refresh natively via `pull: always`).

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/5191.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

The `matrix-client-commet` role's pull task references
`matrix_client_commet_container_image_force_pull`, but the variable
was never defined in defaults. Users setting
`matrix_client_commet_container_image_self_build: false` would hit an
undefined-variable failure.

Define it using the same pattern as other client roles (force-pull on
moving tags), covering both `:latest` and `:main` since Commet's
default version is `main`.