fix: resolve broken alembic migrations (#62) (#63)

- Replace shell variable interpolation in .env.example with literal
  values, since .env files don't expand shell variables
- Fix alembic/env.py to read DATABASE_URL instead of SQLALCHEMY_DATABASE_URL
  to match the app's environment variable convention

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)