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.

lib/openapi.test.ts

@@ -0,0 +1,27 @@
+import pkg from '@/package.json';
+import { describe, expect, it } from 'vitest';
+import { openApiSpec } from './openapi';
+
+describe('OpenAPI spec', () => {
+  it('documents exactly the one public endpoint (scaffold stage)', () => {
+    expect(Object.keys(openApiSpec.paths).sort()).toEqual(['/api/version']);
+  });
+
+  it('keeps info.version in sync with package.json', () => {
+    expect(openApiSpec.info.title).toBe('SpaceWanderer API');
+    expect(openApiSpec.info.version).toBe(pkg.version);
+  });
+
+  it('uses an empty server url so "try it" targets the current origin', () => {
+    expect(openApiSpec.servers).toEqual([{ url: '' }]);
+  });
+
+  it('describes /api/version as { version } with a string version', () => {
+    const schema = openApiSpec.paths['/api/version'].get.responses[200].content['application/json'].schema;
+    expect(schema.$ref).toBe('#/components/schemas/AppVersion');
+
+    const appVersion = openApiSpec.components.schemas.AppVersion.properties;
+    expect(Object.keys(appVersion)).toEqual(['version']);
+    expect(appVersion.version.type).toBe('string');
+  });
+});