Complete backend verification: fix syntax errors, create __init__.py files, add setup_db.py, update docker-compose for two PostgreSQL containers

backend/app/schemas/__init__.py

@@ -0,0 +1 @@
+# Schemas package

backend/app/schemas/protocol_constants.py

@@ -1,4 +1,4 @@
-"""Cockatrice protocol constants and message definitions."""
+"""MTG Online protocol constants and message definitions."""
 import enum