fix(trips): explicit varchar type on cancelled_by (union type broke TypeORM metadata)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/modules/trips/entities/trip.entity.ts

@@ -82,7 +82,7 @@ export class Trip {
   @Column({ default: 'cash' })
   payment_method: string;
 
-  @Column({ nullable: true })
+  @Column({ type: 'varchar', nullable: true })
   cancelled_by: string | null; // rider | driver
 
   @Column({ type: 'numeric', precision: 12, scale: 3, nullable: true })