fix: abandon dispatch when heartbeat loses the lease

Heartbeat renew failures were only logged, so under DB flakiness a job could
be leased twice concurrently — the original worker kept crawling past a lapsed
lease while another re-leased it. Track consecutive renew failures and, after
MAX_HEARTBEAT_RENEW_FAILURES, signal the worker (via a CancellationToken) to
abandon the in-flight dispatch instead of working a job it no longer owns.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.18",
+  "version": "0.124.19",
   "private": true,
   "type": "module",
   "scripts": {