syn2mas reads Synapse's homeserver.yaml and reuses the database
connection details from there.

When Synapse is configured to reach the integrated Postgres over a UNIX socket,
the temporary syn2mas container was given the config file but not the socket mount,
so migrations could fail even though Synapse itself was configured correctly.

Wire the Synapse socket settings into MAS via playbook vars and mount
the same socket path into the syn2mas container, so migrations work in
socket-based deployments without coupling the MAS role directly to
Synapse role variables.