fix(api): remove TimestampMixin from models whose DB tables lack timestamp columns

Remove TimestampMixin (created_at/updated_at) from Purchase, PurchaseItem,
PriceHistory, Coupon, and ShrinkflationEvent models since their PostgreSQL
tables do not have those columns. This was causing 500 errors on
/api/v1/purchases and /api/v1/purchases/stats.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/models/coupon.py

@@ -9,14 +9,14 @@ from sqlalchemy import Boolean, Date, DateTime, ForeignKey, Numeric, String
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from cartsnitch_api.constants import DiscountType
-from cartsnitch_api.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+from cartsnitch_api.models.base import Base, UUIDPrimaryKeyMixin
 
 if TYPE_CHECKING:
     from cartsnitch_api.models.product import NormalizedProduct
     from cartsnitch_api.models.store import Store
 
 
-class Coupon(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+class Coupon(UUIDPrimaryKeyMixin, Base):
     """A coupon or deal for a product at a store."""
 
     __tablename__ = "coupons"