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/app.module.ts

@@ -19,6 +19,7 @@ import { AiModule } from './ai/ai.module';
 import { RealtimeModule } from './realtime/realtime.module';
 import { HelpTextsModule } from './help-texts/help-texts.module';
 import { FlyerImportModule } from './flyer-import/flyer-import.module';
+import { FlyerSelectionModule } from './flyer-selection/flyer-selection.module';
 import { JwtAuthGuard } from './auth/jwt-auth.guard';
 import { RolesGuard } from './auth/roles.guard';
 
@@ -50,6 +51,7 @@ import { RolesGuard } from './auth/roles.guard';
     RealtimeModule,
     HelpTextsModule,
     FlyerImportModule,
+    FlyerSelectionModule,
   ],
   providers: [
     {