Rename metadata fields for clarity and fix imports.
Updated metadata-related field names across models to improve clarity and consistency (e.g., `metadata` to `media_metadata` and `notification_metadata`). Fixed an error in `guest_gifts` metadata reference and added a proper imports initialization in `__init__.py` for all models.
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Introduce EventMedia for managing media files linked to events, including file metadata, purpose, and relationships. Add GiftCategory to categorize gifts, featuring unique constraints, display options, and utility methods for handling related gifts.
