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/bills/dto/bill.dto.ts

@@ -0,0 +1,14 @@
+import { IsString, IsOptional } from 'class-validator';
+
+export class GenerateBillsDto {
+  @IsString()
+  periodStart: string; // YYYY-MM-DD
+
+  @IsString()
+  periodEnd: string; // YYYY-MM-DD
+}
+
+export class UpdateBillStatusDto {
+  @IsString()
+  status: 'draft' | 'confirmed' | 'paid';
+}