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-selection/flyer-selection.module.ts

@@ -0,0 +1,11 @@
+import { Module } from '@nestjs/common';
+import { PrismaModule } from '../prisma/prisma.module';
+import { FlyerSelectionController } from './flyer-selection.controller';
+import { FlyerSelectionService } from './flyer-selection.service';
+
+@Module({
+  imports: [PrismaModule],
+  controllers: [FlyerSelectionController],
+  providers: [FlyerSelectionService],
+})
+export class FlyerSelectionModule {}