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>

These variables track whether a database migration necessitates a service
restart. The new name avoids confusion with the conditional restart
feature introduced in af193043/9accc848/4a8df138, where
devture_systemd_service_manager handles restarting services whose
configuration or image changed. The old _requires_restart name was
ambiguous — it could be mistaken for the systemd_service_manager
mechanism — so _migration_requires_restart makes the purpose explicit.

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

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