feat(loyalty): multi-select categories on transactions

Switch from single category_id to category_ids JSON array on
transactions. Sellers can now select multiple categories (e.g.,
Men + Accessories) when entering stamp/points transactions.

- Migration loyalty_009: drop category_id FK, add category_ids JSON
- Schemas: category_id → category_ids (list[int] | None)
- Services: stamp_service + points_service accept category_ids
- Terminal UI: pills are now multi-select (toggle on/off)
- Transaction response: category_names (list[str]) resolved from IDs
- Recent transactions table: new Category column showing comma-
  separated names

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

app/modules/loyalty/services/stamp_service.py

@@ -46,7 +46,7 @@ class StampService:
         qr_code: str | None = None,
         card_number: str | None = None,
         staff_pin: str | None = None,
-        category_id: int | None = None,
+        category_ids: list[int] | None = None,
         ip_address: str | None = None,
         user_agent: str | None = None,
         notes: str | None = None,
@@ -144,7 +144,7 @@ class StampService:
             card_id=card.id,
             store_id=store_id,
             staff_pin_id=verified_pin.id if verified_pin else None,
-            category_id=category_id,
+            category_ids=category_ids,
             transaction_type=TransactionType.STAMP_EARNED.value,
             stamps_delta=1,
             stamps_balance_after=card.stamp_count,