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

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

server.js

@@ -8,7 +8,7 @@ import { registerWorkers } from './lib/jobs/index.js';
 import { router as ingestRouter } from './lib/api/routes/ingest.js';
 import { startCron } from './lib/cron/index.js';
 
-const VERSION = '2.0.0-alpha.3';
+const VERSION = '2.0.0-alpha.4';
 
 export function createApp() {
   const app = express();