Wire up the MAS=1 environment variable in complement.sh: when set, export
PASS_SYNAPSE_COMPLEMENT_USE_MAS=true and force postgres (MAS requires it
for its own database).
Add PLAN.md documenting the MAS-in-Complement architecture: how nginx
routes traffic between Synapse, MAS, and the registration shim, what
endpoints Complement expects, and how the shim bridges the gap.