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

apps/backend/prisma/migrations/20260617132653_add_wallet_id_to_expense/migration.sql

@@ -0,0 +1,8 @@
+-- AlterTable
+ALTER TABLE "expenses" ADD COLUMN     "walletId" INTEGER;
+
+-- CreateIndex
+CREATE INDEX "expenses_walletId_idx" ON "expenses"("walletId");
+
+-- AddForeignKey
+ALTER TABLE "expenses" ADD CONSTRAINT "expenses_walletId_fkey" FOREIGN KEY ("walletId") REFERENCES "wallets"("id") ON DELETE SET NULL ON UPDATE CASCADE;