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/vitest.config.ts

@@ -0,0 +1,13 @@
+import { defineConfig } from 'vitest/config';
+import tsconfigPaths from 'vite-tsconfig-paths';
+
+export default defineConfig({
+  // Resolves the path aliases declared in tsconfig.json, including the ones
+  // added by `nest g library`.
+  plugins: [tsconfigPaths()],
+  test: {
+    globals: true,
+    root: './',
+    include: ['**/*.spec.ts'],
+  },
+});