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/.oxlintrc.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "./node_modules/oxlint/configuration_schema.json",
+  "rules": {
+    "typescript/no-explicit-any": "off",
+    "typescript/no-floating-promises": "error"
+  },
+  "env": {
+    "node": true
+  }
+}