Part of adopting a uniform naming policy for bridge variables,
where the variable prefix matches the role directory name.

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

Unlike the other mautrix bridges, the mautrix-slack role had no Traefik
label infrastructure at all, so this builds the scaffold first (a new
labels.j2, the container_labels_traefik_* vars, the label-file wiring in
the systemd service and setup_install.yml, and the group_vars wiring),
then exposes the bridge's appservice HTTP API under
https://matrix.<domain>/bridges/slack like the other bridges.

The provisioning shared secret was already auto-generated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>