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/static/store/js/loyalty-terminal.js

@@ -31,7 +31,7 @@ function storeLoyaltyTerminal() {
         // Transaction inputs
         earnAmount: null,
         selectedReward: '',
-        selectedCategory: null,
+        selectedCategories: [],
         categories: [],
 
         // PIN entry
@@ -300,7 +300,7 @@ function storeLoyaltyTerminal() {
             await apiClient.post('/store/loyalty/stamp', {
                 card_id: this.selectedCard.id,
                 staff_pin: this.pinDigits,
-                category_id: this.selectedCategory || undefined,
+                category_ids: this.selectedCategories.length > 0 ? this.selectedCategories : undefined,
             });
 
             Utils.showToast(I18n.t('loyalty.store.terminal.stamp_added'), 'success');
@@ -326,7 +326,7 @@ function storeLoyaltyTerminal() {
                 card_id: this.selectedCard.id,
                 purchase_amount_cents: Math.round(this.earnAmount * 100),
                 staff_pin: this.pinDigits,
-                category_id: this.selectedCategory || undefined,
+                category_ids: this.selectedCategories.length > 0 ? this.selectedCategories : undefined,
             });
 
             const pointsEarned = response.points_earned || Math.floor(this.earnAmount * (this.program?.points_per_euro || 1));