feat(cron): daily sync.source_doc enqueue

node-cron schedules runSync at 03:00 local time; runSync enqueues
sync.source_doc for every source_docs row with sync_source='url'.
Started from server.js's CLI gate alongside the job queue.

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

lib/cron/index.js

@@ -0,0 +1,16 @@
+import cron from 'node-cron';
+import { runSync } from './sync_source_docs.js';
+import { log } from '../log.js';
+
+export function startCron() {
+  // Daily at 03:00 local time
+  cron.schedule('0 3 * * *', async () => {
+    try {
+      const n = await runSync();
+      log.info({ enqueued: n }, 'cron sync.source_doc complete');
+    } catch (e) {
+      log.error({ err: e }, 'cron sync.source_doc failed');
+    }
+  });
+  log.info('cron started');
+}