feat: add student wallet utility billing

apps/server/src/bills/dto/bill.dto.ts

@@ -1,22 +1,28 @@
-import { ArrayNotEmpty, IsArray, IsIn, IsInt, IsOptional, IsString, Matches } from 'class-validator';
+import { ArrayNotEmpty, IsArray, IsIn, IsInt, IsOptional, IsString, Matches, MaxLength } from 'class-validator';
 
 export class GenerateBillsDto {
   @IsString()
   @Matches(/^\d{4}-\d{2}$/)
-  billingMonth: string; // YYYY-MM
+  billingMonth: string;
 
   @IsOptional()
   @IsString()
-  periodStart?: string; // deprecated, derived from billingMonth
+  periodStart?: string;
 
   @IsOptional()
   @IsString()
-  periodEnd?: string; // deprecated, derived from billingMonth
+  periodEnd?: string;
 }
 
 export class UpdateBillStatusDto {
-  @IsIn(['paid'])
-  status: 'paid';
+  @IsIn(['unpaid', 'partially_paid', 'paid'])
+  status: 'unpaid' | 'partially_paid' | 'paid';
+}
+
+export class CancelBillDto {
+  @IsString()
+  @MaxLength(300)
+  reason: string;
 }
 
 export class BatchUpdateBillStatusDto extends UpdateBillStatusDto {