feat(api): app NestJS con configuración validada y health check

Nest 12 (ESM, vitest, oxlint). El .env se valida con zod al arrancar;
prefijo global /api, CORS configurable y GET /api/health.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

apps/api/src/health/health.controller.ts

@@ -0,0 +1,9 @@
+import { Controller, Get } from '@nestjs/common';
+
+@Controller('health')
+export class HealthController {
+  @Get()
+  check() {
+    return { status: 'ok', uptime: Math.round(process.uptime()) };
+  }
+}