feat(expenses): add walletId to expense model and implement wallet adjustments in expense operations

apps/backend/prisma/schema.prisma

@@ -139,6 +139,7 @@ model Wallet {
   updatedAt DateTime  @updatedAt
 
   movements WalletMovement[]
+  expenses  Expense[]
 
   @@map("wallets")
 }
@@ -187,6 +188,9 @@ model Expense {
   status            PaymentStatus    @default(PENDING)
   dueDate           DateTime
   paymentDate       DateTime?
+  walletId          Int?
+  wallet            Wallet?          @relation(fields: [walletId], references: [id])
 
+  @@index([walletId])
   @@map("expenses")
 }