Replace the import-time config with a validated Settings object

[alexhambley]

Issue

• At the moment, configuration is read at import time, via class attributes on Config/DevelopmentConfig/ProductionConfig (app/utils/config.py).
• This is brittle as env vars must be set before the module is imported; FLASK_ENV is read at import.
• Tests have to monkeypatch class attributes (e.g. DevelopmentConfig.MINIO_ENABLED) instead of configuring the app normally.
• Missing or blank required vars surface as runtime 500s (or silently misconfigured Celery) rather than a clear startup failure.