feat: add student wallet utility billing

apps/server/src/entities/wallet-transaction.entity.ts

@@ -0,0 +1,32 @@
+import { Column, CreateDateColumn, Entity, Index, PrimaryGeneratedColumn } from 'typeorm';
+
+@Entity('wallet_transactions')
+@Index(['studentId', 'createdAt'])
+export class WalletTransaction {
+  @PrimaryGeneratedColumn()
+  id: number;
+
+  @Column({ name: 'student_id', type: 'integer' })
+  studentId: number;
+
+  @Column({ name: 'bill_id', type: 'integer', nullable: true })
+  billId: number | null;
+
+  @Column({ type: 'varchar', length: 30 })
+  type: 'recharge' | 'adjustment' | 'bill_payment' | 'bill_refund';
+
+  @Column({ type: 'decimal', precision: 12, scale: 2 })
+  amount: number;
+
+  @Column({ name: 'balance_after', type: 'decimal', precision: 12, scale: 2 })
+  balanceAfter: number;
+
+  @Column({ type: 'varchar', length: 300, nullable: true })
+  description: string | null;
+
+  @Column({ name: 'recorded_by', type: 'integer', nullable: true })
+  recordedBy: number | null;
+
+  @CreateDateColumn({ name: 'created_at' })
+  createdAt: Date;
+}