feat(newsletter): scan forward for multiple issues and re-enable cron

Loop through consecutive newsletter numbers until one is missing, so
backlog gets delivered in a single run. Use httpx for 404 check and
point to absolute claude binary path for cron. Enable job in config.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.json

@@ -49,7 +49,7 @@
     }
   },
   "newsletter_cercetasi": {
-    "enabled": false,
+    "enabled": true,
     "cron": "0 17 * * 4,5,1",
     "channel": "echo-core"
   },