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/routes/api/store.py

@@ -762,7 +762,7 @@ def add_stamp(
         qr_code=data.qr_code,
         card_number=data.card_number,
         staff_pin=data.staff_pin,
-        category_id=data.category_id,
+        category_ids=data.category_ids,
         ip_address=ip,
         user_agent=user_agent,
         notes=data.notes,
@@ -853,7 +853,7 @@ def earn_points(
         purchase_amount_cents=data.purchase_amount_cents,
         order_reference=data.order_reference,
         staff_pin=data.staff_pin,
-        category_id=data.category_id,
+        category_ids=data.category_ids,
         ip_address=ip,
         user_agent=user_agent,
         notes=data.notes,