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/shrinkflation.py

@@ -9,13 +9,13 @@ from sqlalchemy import Date, ForeignKey, Numeric, String
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from cartsnitch_api.constants import SizeUnit
-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
 
 
-class ShrinkflationEvent(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+class ShrinkflationEvent(UUIDPrimaryKeyMixin, Base):
     """Detected shrinkflation event — product size changed while price held or rose."""
 
     __tablename__ = "shrinkflation_events"