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/schemas/stamp.py

@@ -37,10 +37,10 @@ class StampRequest(BaseModel):
         description="Staff PIN for verification",
     )
 
-    # Category (what was sold)
-    category_id: int | None = Field(
+    # Categories (what was sold — multi-select)
+    category_ids: list[int] | None = Field(
         None,
-        description="Transaction category ID (e.g., Men, Women, Accessories)",
+        description="Transaction category IDs",
     )
 
     # Optional metadata