fix: exponential backoff for idle crawler workers

An idle worker re-issued a row-locking lease query every 1s per worker with
no backoff. Replace the flat sleep with idle_backoff: 1s, 2s, 4s, … capped at
30s, reset to 0 the moment a job is leased, so a quiet daemon stops hammering
the crawler_jobs table.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.17",
+  "version": "0.124.18",
   "private": true,
   "type": "module",
   "scripts": {