feat(ai): add AI trace tracking and admin panel

- Add AiTrace model to Prisma schema with relations to User
- Implement AiTraceService with CRUD operations for AI traces
- Add new admin panel for AI traces with filtering and detail views
- Integrate trace persistence in receipt import flow
- Add API endpoints for listing and retrieving AI traces
- Update Flutter admin UI with new AI tab and navigation
- Add new domain models for AI traces and details
- Add migration for AiTrace table creation

BREAKING CHANGE: None

backend/prisma/schema.prisma

@@ -33,6 +33,7 @@ model User {
   flyerSessions    FlyerSession[]
   flyerSelections  FlyerSelection[]
   shoppingListItems ShoppingListItem[]
+  aiTraces          AiTrace[]
 }
 
 model Product {
@@ -388,3 +389,25 @@ model ShoppingListItem {
   @@index([productId, unit, status])
   @@index([categoryId])
 }
+
+model AiTrace {
+  id               Int      @id @default(autoincrement())
+  source           String
+  userId           Int?
+  sessionId        Int?
+  model            String?
+  prompt           String?  @db.LongText
+  rawOutput        String?  @db.LongText
+  normalizedOutput Json?
+  status           String
+  error            String?  @db.Text
+  durationMs       Int?
+  createdAt        DateTime @default(now())
+  updatedAt        DateTime @updatedAt
+
+  user             User?    @relation(fields: [userId], references: [id], onDelete: SetNull)
+
+  @@index([source, createdAt])
+  @@index([userId, createdAt])
+  @@index([status, createdAt])
+}