Emit a /.well-known/matrix/mautrix file listing the base URLs of all
enabled and exposed mautrix bridges under the `fi.mau.bridges` property,
so tools like Mautrix Manager (https://github.com/mautrix/manager) can
auto-discover them.

The list is built in group_vars from each bridge's public address and is
gated on the bridge being enabled, the playbook attaching its Traefik
labels, and the exposure router being emitted, so we only advertise URLs
that are actually reachable. The file follows the same auto/custom and
configuration-extension pattern as the other well-known files and is only
written when the list is non-empty.

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