feat(loyalty): transaction categories (what was sold)

Merchants can configure per-store product categories (e.g., Men,
Women, Accessories, Kids) that sellers select when entering loyalty
transactions. Enables per-category sales analytics.

Backend:
- New model: StoreTransactionCategory (store-scoped, max 10 per store)
- Migration loyalty_007: creates table + adds category_id FK on
  loyalty_transactions
- New category_service.py with CRUD + validation
- New schemas/category.py (Create, Update, Response, ListResponse)
- Admin CRUD: GET/POST/PATCH/DELETE /admin/loyalty/stores/{id}/categories
- Store CRUD: GET/POST/PATCH/DELETE /store/loyalty/categories
- Stamp/Points request schemas accept optional category_id
- Stamp/Points services pass category_id to transaction creation
- TransactionResponse includes category_id + category_name

342 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/models/__init__.py

@@ -49,6 +49,10 @@ from app.modules.loyalty.models.staff_pin import (
     # Model
     StaffPin,
 )
+from app.modules.loyalty.models.transaction_category import (
+    # Model
+    StoreTransactionCategory,
+)
 
 __all__ = [
     # Enums
@@ -62,4 +66,5 @@ __all__ = [
     "StaffPin",
     "AppleDeviceRegistration",
     "MerchantLoyaltySettings",
+    "StoreTransactionCategory",
 ]