api: /api/version + OpenAPI 3.x spec generated in code

lib/openapi.ts is the machine-readable spec (served at /api/openapi) per AGENTS.md;
info.version sourced from package.json. Unit tests pin the spec shape and version sync.

app/api/openapi/route.ts

@@ -0,0 +1,5 @@
+import { openApiSpec } from '@/lib/openapi';
+
+export async function GET() {
+  return Response.json(openApiSpec);
+}

app/api/version/route.ts

@@ -0,0 +1,5 @@
+import pkg from '@/package.json';
+
+export async function GET() {
+  return Response.json({ version: pkg.version });
+}