```

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.
```

backend/app/models/guest.py

@@ -84,7 +84,7 @@ class Guest(Base, UUIDMixin, TimestampMixin):
 # Association table for guest gifts (many-to-many relationship)
 guest_gifts = Table(
     'guest_gifts',
-    Base.metadata,
+    Base.notification_metadata,
     Column('guest_id', UUID(as_uuid=True), ForeignKey('guests.id'), primary_key=True),
     Column('gift_id', UUID(as_uuid=True), ForeignKey('gift_items.id'), primary_key=True),
     Column('reserved_at', DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)),