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/crud/gift.py

@@ -94,13 +94,14 @@ class CRUDGiftItem(CRUDBase[GiftItem, GiftItemCreate, GiftItemUpdate]):
         gift: GiftItem = self.get(db, gift_id)
         if gift and gift.quantity_received < gift.quantity_requested:
             # Add to the association table using the SQLAlchemy Core Table directly
-            from app.models.guest import guest_gifts
+            from app.models.guest import GuestGifts
 
-            stmt = guest_gifts.insert().values(
+            stmt = GuestGifts.insert().values(
                 gift_id=gift_id,
                 guest_id=guest_id,
                 reserved_at=datetime.now(timezone.utc),
-                notes=notes
+                notes=notes,
+                quantity=quantity
             )
             db.execute(stmt)
 
@@ -336,12 +337,12 @@ class CRUDGiftPurchase(CRUDBase[GiftPurchase, GiftPurchaseCreate, GiftPurchaseUp
         for gift in guest.gifts:
             # Access the association data through the relationship metadata
             from sqlalchemy import select
-            from app.models.guest import guest_gifts
+            from app.models.guest import GuestGifts
 
             # Get the reservation data
-            stmt = select(guest_gifts).where(
-                (guest_gifts.c.guest_id == guest_id) &
-                (guest_gifts.c.gift_id == gift.id)
+            stmt = select(GuestGifts).where(
+                (GuestGifts.c.guest_id == guest_id) &
+                (GuestGifts.c.gift_id == gift.id)
             )
             reservation = db.execute(stmt).first()