feat: add student wallet utility billing

apps/server/src/expenses/dto/expense.dto.ts

@@ -1,4 +1,4 @@
-import { IsInt, IsString, IsNumber, IsOptional } from 'class-validator';
+import { IsIn, IsInt, IsString, IsNumber, IsOptional, Matches, Min } from 'class-validator';
 
 export class CreateRoomExpenseDto {
   @IsInt()
@@ -52,3 +52,28 @@ export class BatchRoomExpenseDto {
 
   expenses: { roomId: number; expenseType: string; amount: number; description?: string }[];
 }
+
+
+export class CreateStudentUtilityBillDto {
+  @IsInt()
+  studentId: number;
+
+  @IsIn(['water', 'electricity'])
+  expenseType: 'water' | 'electricity';
+
+  @IsNumber({ maxDecimalPlaces: 2 })
+  @Min(0.01)
+  amount: number;
+
+  @IsString()
+  @Matches(/^\d{4}-\d{2}-\d{2}$/)
+  periodStart: string;
+
+  @IsString()
+  @Matches(/^\d{4}-\d{2}-\d{2}$/)
+  periodEnd: string;
+
+  @IsOptional()
+  @IsString()
+  description?: string;
+}