Add MAS=1 integration test support to complement.sh

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.

scripts-dev/complement.sh

@@ -362,6 +362,13 @@ main() {
     export PASS_SYNAPSE_USE_UNIX_SOCKET=1
   fi
 
+  if [[ -n "$MAS" ]]; then
+    # Enable Matrix Authentication Service (MAS) integration.
+    # MAS requires postgres for its own database.
+    export PASS_SYNAPSE_COMPLEMENT_USE_MAS=true
+    export PASS_SYNAPSE_COMPLEMENT_DATABASE=postgres
+  fi
+
   if [[ -n "$SYNAPSE_TEST_LOG_LEVEL" ]]; then
     # Set the log level to what is desired
     export PASS_SYNAPSE_LOG_LEVEL="$SYNAPSE_TEST_LOG_LEVEL"