fix: resolve .env.example variable interpolation and alembic env var mismatch (#62)

.env files don't support shell variable expansion, so DATABASE_URL and
REDIS_URL contained literal ${...} strings. Also alembic/env.py read
SQLALCHEMY_DATABASE_URL instead of DATABASE_URL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

alembic/env.py

@@ -19,7 +19,7 @@ if config.config_file_name is not None:
 target_metadata = Base.metadata
 
 # Get database URL from environment variable or config
-database_url = os.environ.get("SQLALCHEMY_DATABASE_URL")
+database_url = os.environ.get("DATABASE_URL")
 if database_url:
     config.set_main_option("sqlalchemy.url", database_url)