feat: add CategoriesModule with CRUD operations

Implemented CategoriesModule with controller, service, and DTOs for managing categories. Includes endpoints for creation, retrieval, updating, and deletion, integrated with database logic.

backend/src/categories/dto/create-category.dto.ts

@@ -0,0 +1,15 @@
+import { IsNotEmpty, IsOptional, IsString } from "class-validator";
+
+export class CreateCategoryDto {
+	@IsString()
+	@IsNotEmpty()
+	name!: string;
+
+	@IsOptional()
+	@IsString()
+	description?: string;
+
+	@IsOptional()
+	@IsString()
+	iconUrl?: string;
+}

backend/src/categories/dto/update-category.dto.ts

@@ -0,0 +1,4 @@
+import { PartialType } from "@nestjs/mapped-types";
+import { CreateCategoryDto } from "./create-category.dto";
+
+export class UpdateCategoryDto extends PartialType(CreateCategoryDto) {}