feat(task1): restructure directories for turborepo monorepo

- Move backend/ to apps/server/ via git mv
- Move frontend/ to apps/admin/ via git mv
- Create packages/typescript-config/ with base, nestjs, and react-vite presets

apps/server/src/entities/bill.entity.ts

@@ -0,0 +1,40 @@
+import { Entity, PrimaryGeneratedColumn, Column, CreateDateColumn, ManyToOne, JoinColumn, OneToMany } from 'typeorm';
+import { Student } from './student.entity';
+import { BillItem } from './bill-item.entity';
+
+@Entity('bills')
+export class Bill {
+  @PrimaryGeneratedColumn()
+  id: number;
+
+  @Column({ name: 'student_id' })
+  studentId: number;
+
+  @Column({ name: 'period_start', type: 'date' })
+  periodStart: string;
+
+  @Column({ name: 'period_end', type: 'date' })
+  periodEnd: string;
+
+  @Column({ name: 'shared_amount', type: 'decimal', precision: 10, scale: 2, default: 0 })
+  sharedAmount: number;
+
+  @Column({ name: 'personal_amount', type: 'decimal', precision: 10, scale: 2, default: 0 })
+  personalAmount: number;
+
+  @Column({ name: 'total_amount', type: 'decimal', precision: 10, scale: 2, default: 0 })
+  totalAmount: number;
+
+  @Column({ type: 'varchar', length: 20, default: 'draft' })
+  status: string;
+
+  @CreateDateColumn({ name: 'generated_at' })
+  generatedAt: Date;
+
+  @ManyToOne(() => Student, (s) => s.bills)
+  @JoinColumn({ name: 'student_id' })
+  student: Student;
+
+  @OneToMany(() => BillItem, (bi) => bi.bill)
+  items: BillItem[];
+}