Add quantity column to GuestGifts table and update references

Introduced a `quantity` field to track the number of reserved gifts in the many-to-many GuestGifts table. Updated all related CRUD operations, models, and imports to reflect the added column. Replaced `guest_gifts` with `GuestGifts` for consistency in naming conventions.

backend/app/models/guest.py

@@ -82,11 +82,12 @@ class Guest(Base, UUIDMixin, TimestampMixin):
 
 
 # Association table for guest gifts (many-to-many relationship)
-guest_gifts = Table(
+GuestGifts = Table(
     'guest_gifts',
     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)),
     Column('notes', String),
+    Column('quantity', Integer, default=1),
 )