fix: harden financial transaction boundaries

apps/server/src/entities/room-expense.entity.ts

@@ -5,10 +5,12 @@ import {
   CreateDateColumn,
   ManyToOne,
   JoinColumn,
+  Index,
 } from 'typeorm';
 import { Room } from './room.entity';
 
 @Entity('room_expenses')
+@Index(['importKey'], { unique: true })
 export class RoomExpense {
   @PrimaryGeneratedColumn()
   id: number;
@@ -34,6 +36,9 @@ export class RoomExpense {
   @Column({ name: 'recorded_by', nullable: true })
   recordedBy: number;
 
+  @Column({ name: 'import_key', type: 'varchar', length: 120, nullable: true, unique: true })
+  importKey: string | null;
+
   @Column({ type: 'varchar', length: 20, default: 'active' })
   status: 'active' | 'archived';