chore: version 2.0.0-alpha.4 + changelog + plan-4 completion doc

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/server.test.js

@@ -17,7 +17,7 @@ describe('server', () => {
     const res = await request(app).get('/health');
     expect(res.status).toBe(200);
     expect(res.body.db_ok).toBe(true);
-    expect(res.body.version).toBe('2.0.0-alpha.3');
+    expect(res.body.version).toBe('2.0.0-alpha.4');
   });
 
   it('GET /api/spaces without token returns 401', async () => {