Add conditional restart support to client, bot, and bridge roles

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>

roles/custom/matrix-bridge-mautrix-telegram/defaults/main.yml

@@ -258,3 +258,13 @@ matrix_mautrix_telegram_displayname_template: '{displayname} (Telegram)'
 matrix_mautrix_telegram_bridge_encryption_allow: "{{ matrix_bridges_encryption_enabled }}"
 matrix_mautrix_telegram_bridge_encryption_default: "{{ matrix_bridges_encryption_default }}"
 matrix_mautrix_telegram_bridge_encryption_key_sharing_allow: "{{ matrix_mautrix_telegram_bridge_encryption_allow }}"
+
+# matrix_mautrix_telegram_restart_necessary controls whether the service
+# will be restarted (when true) or merely started (when false) by the
+# systemd service manager role (when conditional restart is enabled).
+#
+# This value is automatically computed during installation based on whether
+# any configuration files, the systemd service file, or the container image changed.
+# The default of `false` means "no restart needed" — appropriate when the role's
+# installation tasks haven't run (e.g., due to --tags skipping them).
+matrix_mautrix_telegram_restart_necessary: false