refactor: consolidate runtime helpers

src/message-cursor.ts

@@ -0,0 +1,10 @@
+import { getLatestMessageSeqAtOrBefore } from './db.js';
+
+export function normalizeStoredSeqCursor(
+  cursor: string | undefined,
+  chatJid?: string,
+): string {
+  if (!cursor) return '0';
+  if (/^\d+$/.test(cursor.trim())) return cursor.trim();
+  return String(getLatestMessageSeqAtOrBefore(cursor, chatJid));
+}