chore(repo): restructure as monorepo with server and client workspaces

- move NestJS app into server/ subdirectory
- add client/ React+TypeScript (Vite) app with Hello World
- update docker-compose to build and run both services
- add root package.json declaring npm workspaces
- update .gitignore to cover node_modules and dist at all depths

server/src/scenario/scenario.entity.ts

@@ -0,0 +1,30 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  CreateDateColumn,
+  UpdateDateColumn,
+  OneToMany,
+} from "typeorm";
+import { ScenarioStepEntity } from "./scenario-step.entity";
+
+@Entity("scenarios")
+export class ScenarioEntity {
+  @PrimaryGeneratedColumn()
+  id: number;
+
+  @Column()
+  name: string;
+
+  @OneToMany(() => ScenarioStepEntity, (step) => step.scenario, {
+    cascade: true,
+    eager: false,
+  })
+  steps: ScenarioStepEntity[];
+
+  @CreateDateColumn()
+  createdAt: Date;
+
+  @UpdateDateColumn()
+  updatedAt: Date;
+}