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/prisma/schema.prisma

@@ -27,10 +27,12 @@ model User {
   ownedProducts Product[]
   inventoryItems InventoryItem[]
   pantryItems PantryItem[]
-  mealPlanEntries MealPlanEntry[]
-  receiptAliases  ReceiptAlias[]
-  unitMappings     UnitMapping[]
-}
+  mealPlanEntries MealPlanEntry[]
+  receiptAliases  ReceiptAlias[]
+  unitMappings     UnitMapping[]
+  flyerSessions    FlyerSession[]
+  flyerSelections  FlyerSelection[]
+}
 
 model Product {
   id             Int             @id @default(autoincrement())
@@ -264,7 +266,7 @@ model UnitMapping {
   @@index([userId])
 }
 
-model HelpText {
+model HelpText {
   id        Int      @id @default(autoincrement())
   key       String
   scope     String   @default("default")
@@ -274,6 +276,75 @@ model HelpText {
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
-  @@unique([key, scope])
-  @@index([key, isActive])
-}
+  @@unique([key, scope])
+  @@index([key, isActive])
+}
+
+model FlyerSession {
+  id         Int              @id @default(autoincrement())
+  userId     Int
+  retailer   String
+  weekKey    String
+  status     String           @default("draft")
+  createdAt  DateTime         @default(now())
+  updatedAt  DateTime         @updatedAt
+  expiresAt  DateTime?
+
+  user       User             @relation(fields: [userId], references: [id], onDelete: Cascade)
+  items      FlyerItem[]
+  selections FlyerSelection[]
+
+  @@index([userId])
+  @@index([weekKey])
+  @@index([status])
+}
+
+model FlyerItem {
+  id                 Int              @id @default(autoincrement())
+  sessionId          Int
+  rawName            String
+  normalizedName     String
+  categoryHint       String?
+  price              Decimal?         @db.Decimal(10, 2)
+  priceUnit          String?
+  comparisonPrice    Decimal?         @db.Decimal(10, 2)
+  comparisonUnit     String?
+  offerText          String?
+  parseConfidence    Float
+  parseReasons       Json?
+  matchedProductId   Int?
+  matchedProductName String?
+  matchedVia         String?
+  matchConfidence    Float?
+  matchReasons       Json?
+  createdAt          DateTime         @default(now())
+  updatedAt          DateTime         @updatedAt
+
+  session            FlyerSession     @relation(fields: [sessionId], references: [id], onDelete: Cascade)
+  selections         FlyerSelection[]
+
+  @@index([sessionId])
+  @@index([normalizedName])
+}
+
+model FlyerSelection {
+  id              Int          @id @default(autoincrement())
+  sessionId       Int
+  itemId          Int
+  userId          Int
+  plannedQuantity Decimal?     @db.Decimal(10, 2)
+  plannedUnit     String?
+  priority        String       @default("normal")
+  note            String?
+  status          String       @default("planned")
+  createdAt       DateTime     @default(now())
+  updatedAt       DateTime     @updatedAt
+
+  session         FlyerSession @relation(fields: [sessionId], references: [id], onDelete: Cascade)
+  item            FlyerItem    @relation(fields: [itemId], references: [id], onDelete: Cascade)
+  user            User         @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([sessionId, itemId])
+  @@index([sessionId])
+  @@index([userId, status])
+}