For each of the 34 roles (3 clients, 9 bots, 22 bridges), this commit:
- Adds `_restart_necessary: false` default variable
- Adds `register:` directives to config/image/systemd tasks
- Computes `_restart_necessary` via set_fact (OR of all .changed results)
- Wires `(_restart_necessary | bool)` in group_vars/matrix_servers

This allows the systemd service manager to skip unnecessary restarts
when running install-* tags and nothing actually changed.

Service roles and complex multi-service roles will follow separately.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Closes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4039

Partially reverts 30dad8ba27 which renamed
`config.yml` to `config.yaml` in the playbook and on the server, for
consistency with the rest of the playbook.

The problem is that:

- baibot defaults to looking for `config.yml`, not `config.yaml` (as provided).
  This can be worked around by specifying a new `BAIBOT_CONFIG_FILE_PATH=config.yaml`
  environment variable. This brings more complexity.

- renaming the target file (on the server) to `config.yaml` means people
  with an existing installation would drag around the old file (`config.yml`) as well,
  unless we create a new Ansible task (`ansible.builtin.file` with `state: absent`) to remove
  the old file. This brings more complexity as well.

https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4039 adjusts where the file is mounted,
which fixes the immediate problem (baibot not starting), but still means
people will end up with 2 config files for baibot (`config.yml` and `config.yaml`).

This patch, reverts a bit more, so that we still continue to use `config.yml` on the server.
People who have upgraded within the last ~17 hours may end up with 2 files, but it shouldn't be too many of them.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3369