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/migrations/versions/loyalty_009_category_id_to_category_ids.py

@@ -0,0 +1,43 @@
+"""loyalty 009 - replace category_id FK with category_ids JSON
+
+Switches from single-category to multi-category support on transactions.
+Not live yet so no data migration needed.
+
+Revision ID: loyalty_009
+Revises: loyalty_008
+Create Date: 2026-04-19
+"""
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "loyalty_009"
+down_revision = "loyalty_008"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.drop_column("loyalty_transactions", "category_id")
+    op.add_column(
+        "loyalty_transactions",
+        sa.Column(
+            "category_ids",
+            sa.JSON(),
+            nullable=True,
+            comment="List of category IDs selected for this transaction",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("loyalty_transactions", "category_ids")
+    op.add_column(
+        "loyalty_transactions",
+        sa.Column(
+            "category_id",
+            sa.Integer(),
+            sa.ForeignKey("store_transaction_categories.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )