Refuse to boot without secrets

Implement load_config(env, required=('SECRET_KEY', 'DB_URL')): - returns a dict of every key in required that is present AND non-empty (stripped) - missing or empty required keys → raise ConfigError('missing required config: ' + ', '.join(sorted(missing))) - extra keys in env are IGNORED (never trusted into the config) - define class ConfigError(Exception)

Difficulty: advanced