feat(flyer-import): add bundle support and new product fields

- Add bundle support with isBundle, bundleWeight, and bundleItems fields
- Add brand, weight, and comparisonUnit fields to FlyerItem model
- Update AI flyer parser to extract bundle information
- Add sanitization for bundle items in FlyerNormalizerService
- Update DTOs and interfaces to include new fields
- Add migration for new database fields
- Update tests to cover bundle item handling

backend/prisma/migrations/20260521131500_add_bundle_fields_to_flyer_item/migration.sql

@@ -0,0 +1,7 @@
+-- AlterTable
+ALTER TABLE `FlyerItem`
+  ADD COLUMN `brand` VARCHAR(191) NULL,
+  ADD COLUMN `weight` VARCHAR(191) NULL,
+  ADD COLUMN `bundleWeight` VARCHAR(191) NULL,
+  ADD COLUMN `isBundle` BOOLEAN NOT NULL DEFAULT false,
+  ADD COLUMN `bundleItems` JSON NULL;