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/services/points_service.py

@@ -48,6 +48,7 @@ class PointsService:
         purchase_amount_cents: int,
         order_reference: str | None = None,
         staff_pin: str | None = None,
+        category_id: int | None = None,
         ip_address: str | None = None,
         user_agent: str | None = None,
         notes: str | None = None,
@@ -181,6 +182,7 @@ class PointsService:
             card_id=card.id,
             store_id=store_id,
             staff_pin_id=verified_pin.id if verified_pin else None,
+            category_id=category_id,
             transaction_type=TransactionType.POINTS_EARNED.value,
             points_delta=points_earned,
             stamps_balance_after=card.stamp_count,