fix: add missing relationship stubs to resolve SQLAlchemy mapper errors

- Purchase.user relationship to satisfy User.purchases back_populates
- Store.user_accounts relationship to satisfy UserStoreAccount.store back_populates

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

src/receiptwitness/shared/models/stub_purchase.py

@@ -9,6 +9,8 @@ import uuid
 from datetime import date, datetime
 from decimal import Decimal
 
+from typing import TYPE_CHECKING
+
 from sqlalchemy import (
     JSON,
     Date,
@@ -20,10 +22,13 @@ from sqlalchemy import (
     UniqueConstraint,
     func,
 )
-from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from receiptwitness.shared.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
 
+if TYPE_CHECKING:
+    from receiptwitness.shared.models.user import User
+
 
 class Purchase(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     """Stub: a shopping trip/receipt. Full definition in cartsnitch/common."""
@@ -52,6 +57,8 @@ class Purchase(UUIDPrimaryKeyMixin, TimestampMixin, Base):
         UniqueConstraint("user_id", "store_id", "receipt_id", name="uq_purchase_receipt"),
     )
 
+    user: Mapped["User"] = relationship(back_populates="purchases")
+
 
 class PurchaseItem(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     """Stub: a line item on a receipt. Full definition in cartsnitch/common."""

src/receiptwitness/shared/models/stub_store.py

@@ -7,12 +7,17 @@ UserStoreAccount. The canonical definitions live in cartsnitch/common.
 
 import uuid
 
+from typing import TYPE_CHECKING
+
 from sqlalchemy import Float, ForeignKey, String
-from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from receiptwitness.shared.constants import StoreSlug
 from receiptwitness.shared.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
 
+if TYPE_CHECKING:
+    from receiptwitness.shared.models.user import UserStoreAccount
+
 
 class Store(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     """Stub: canonical retailer. Full definition in cartsnitch/common."""
@@ -24,6 +29,8 @@ class Store(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     logo_url: Mapped[str | None] = mapped_column(String(500))
     website_url: Mapped[str | None] = mapped_column(String(500))
 
+    user_accounts: Mapped[list["UserStoreAccount"]] = relationship(back_populates="store")
+
 
 class StoreLocation(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     """Stub: physical store location. Full definition in cartsnitch/common."""