fix: exponential idle backoff for the analysis worker

The analysis worker's flat 1s idle sleep meant an empty queue fired a lease query
per worker per second forever (audit). Add idle_backoff (1s..30s, reset on lease),
mirroring the crawler. Unit-tested.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.20",
+  "version": "0.128.21",
   "private": true,
   "type": "module",
   "scripts": {