Adds a second Traefik router on the web (nginx) service that listens on
the `matrix-federation` entrypoint (host port 8448 via the traefik
recipe's compose.matrix.yml) and uses Host(${SERVER_NAME}).
Lets remote homeservers reach this server's federation/key endpoints via
the standard `<server_name>:8448` fallback when no .well-known or SRV
delegation exists for SERVER_NAME — useful when SERVER_NAME differs from
DOMAIN and no external service serves /.well-known/matrix/server.
Requires MATRIX_FEDERATION_ENABLED=1 on the traefik stack.
notplants