Refactor models to improve consistency and relationships

Updated `Base` import to use centralized `app.core.database.Base` for consistency. Enhanced `TimestampMixin` and `UUIDMixin` with docstrings for clarity. Fixed metadata reference in `guest_gifts` table and added relationships in `email_template`.

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.notification_metadata,
+    Base.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)),