feat(flyer): add flyer session and selection system

- Add FlyerSession, FlyerItem, and FlyerSelection models to Prisma schema
- Implement session persistence with weekly key generation in FlyerImportService
- Add FlyerSelectionModule to AppModule
- Extend FlyerImportResponse with sessionId and flyerItemId fields
- Create new flyer-selection module directory structure
- Add migration for flyer session and selection tables

BREAKING CHANGE: Flyer import now persists data to FlyerSession and FlyerItem tables

backend/src/flyer-import/dto/flyer-import.response.ts

@@ -1,6 +1,7 @@
 export type FlyerImportMatchVia = 'alias' | 'exact' | 'token' | 'none';
 
 export type FlyerImportItem = {
+  flyerItemId: number | null;
   rawName: string;
   normalizedName: string;
   category: string | null;
@@ -19,6 +20,7 @@ export type FlyerImportItem = {
 };
 
 export type FlyerImportResponse = {
+  sessionId: number | null;
   retailer: 'willys';
   parserVersion: 'v1';
   items: FlyerImportItem[];