refactor: remove S3 dependencies, use Django 5 STORAGES

[edimedia]

BREAKING CHANGE: Removed S3-specific storage configuration in favor of Django 5's STORAGES system

Changes:

• Removed S3 storage helper methods (get_static_storage_url, get_default_storage_url, etc.)
• Added get_log_storage() function that integrates with Django 5 STORAGES
• Updated settings to remove S3-related configuration
• Added model_post_init to OxUtilsSettings for JWT key validation
• Fixed test_jwt_key_validation_file_not_found test

Documentation updates:

• Removed S3 references from audit.md, settings.md, and jwt.md
• Added Storage Integration section to logger.md
• Documented get_log_storage() function and STORAGES configuration
• Updated best practices to remove storage-specific terminology

Migration guide:
Applications should now configure storage backends using Django's STORAGES setting: STORAGES = {
'logs': {
'BACKEND': 'django.core.files.storage.FileSystemStorage',
'OPTIONS': {'location': '/var/log/myapp'},
}
}

Use get_log_storage() to access the configured log storage backend.