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/tsconfig.build.json

@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "rootDir": "./src"
+  },
+  "include": ["src"],
+  "exclude": ["node_modules", "test", "dist", "**/*spec.ts"]
+}