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/models/loyalty_transaction.py

@@ -25,6 +25,7 @@ from sqlalchemy import (
     String,
     Text,
 )
+from sqlalchemy.dialects.sqlite import JSON
 from sqlalchemy.orm import relationship
 
 from app.core.database import Base
@@ -104,11 +105,10 @@ class LoyaltyTransaction(Base, TimestampMixin):
         index=True,
         comment="Staff PIN used for this operation",
     )
-    category_id = Column(
-        Integer,
-        ForeignKey("store_transaction_categories.id", ondelete="SET NULL"),
+    category_ids = Column(
+        JSON,
         nullable=True,
-        comment="Product category (e.g., Men, Women, Accessories)",
+        comment="List of category IDs selected for this transaction",
     )
 
     # Related transaction (for voids/returns)