feat(flyer-import): add detailed product signals and display names

- Added `signals` and `displayNameDetailed` fields to FlyerItem model in Prisma schema
- Introduced `FlyerImportSignals` type with origin countries, labels, quality flags, variant, and packaging
- Added `displayNameDetailed` field to FlyerImportItem DTO and Flutter model
- Implemented utility functions for signal extraction and display name building
- Updated flyer import service to persist and return signals/category data
- Enhanced Flutter UI to display detailed product information including badges for signals
- Added new test coverage for signals persistence and display name generation
- Added new import-common module for shared import utilities
- Created database migration for new fields
- Added Kilo plan for feature development

backend/prisma/migrations/20260524193000_add_flyer_item_signals_and_display_name/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE `FlyerItem`
+  ADD COLUMN `signals` JSON NULL,
+  ADD COLUMN `displayNameDetailed` VARCHAR(191) NULL;

backend/prisma/schema.prisma

@@ -325,6 +325,8 @@ model FlyerItem {
   bundleWeight       String?
   isBundle           Boolean          @default(false)
   bundleItems        Json?
+  signals            Json?
+  displayNameDetailed String?
   offerText          String?
   parseConfidence    Float
   parseReasons       Json?