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/flyer-import.service.ts

@@ -4,6 +4,7 @@ import {
   Logger,
   ServiceUnavailableException,
 } from '@nestjs/common';
+import { Prisma } from '@prisma/client';
 import { PrismaService } from '../prisma/prisma.service';
 import { normalizeName } from '../common/utils/normalize-name';
 import {
@@ -79,6 +80,7 @@ export class FlyerImportService {
     const items: FlyerImportItem[] = parsed.items.map((item) => {
       const match = this.matchItem(item, products, aliasToProduct, productById);
       return {
+        flyerItemId: null,
         rawName: item.rawName,
         normalizedName: item.normalizedName,
         category: item.category,
@@ -97,14 +99,74 @@ export class FlyerImportService {
       };
     });
 
+    const persistedItems = await this.persistSessionWithItems(userId, parsed.retailer, items);
+
     return {
+      sessionId: persistedItems.sessionId,
       retailer: parsed.retailer,
       parserVersion: parsed.parserVersion,
-      items,
+      items: persistedItems.items,
       warnings: parsed.warnings,
     };
   }
 
+  private async persistSessionWithItems(
+    userId: number,
+    retailer: 'willys',
+    items: FlyerImportItem[],
+  ): Promise<{ sessionId: number; items: FlyerImportItem[] }> {
+    const weekKey = this.toWeekKey(new Date());
+
+    const session = await this.prisma.flyerSession.create({
+      data: {
+        userId,
+        retailer,
+        weekKey,
+        status: 'draft',
+      },
+      select: { id: true },
+    });
+
+    const savedItems: FlyerImportItem[] = [];
+    for (const item of items) {
+      const created = await this.prisma.flyerItem.create({
+        data: {
+          sessionId: session.id,
+          rawName: item.rawName,
+          normalizedName: item.normalizedName,
+          categoryHint: item.category,
+          price: item.price != null ? new Prisma.Decimal(item.price) : null,
+          priceUnit: item.priceUnit,
+          comparisonPrice:
+            item.comparisonPrice != null ? new Prisma.Decimal(item.comparisonPrice) : null,
+          comparisonUnit: item.comparisonUnit,
+          offerText: item.offerText,
+          parseConfidence: item.parseConfidence,
+          parseReasons: item.parseReasons,
+          matchedProductId: item.matchedProductId,
+          matchedProductName: item.matchedProductName,
+          matchedVia: item.matchedVia,
+          matchConfidence: item.matchConfidence,
+          matchReasons: item.matchReasons,
+        },
+        select: { id: true },
+      });
+
+      savedItems.push({ ...item, flyerItemId: created.id });
+    }
+
+    return { sessionId: session.id, items: savedItems };
+  }
+
+  private toWeekKey(date: Date): string {
+    const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
+    const dayNum = d.getUTCDay() || 7;
+    d.setUTCDate(d.getUTCDate() + 4 - dayNum);
+    const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
+    const weekNo = Math.ceil((((d.getTime() - yearStart.getTime()) / 86400000) + 1) / 7);
+    return `${d.getUTCFullYear()}-W${String(weekNo).padStart(2, '0')}`;
+  }
+
   private matchItem(
     item: FlyerParseItem,
     products: ProductLite[],