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>

cron/newsletter-cercetasi-state.json

@@ -1 +1,5 @@
-{"last_sent": 13, "year": 2026, "last_sent_at": "2026-04-02T18:18:46.000000+00:00"}
+{
+  "last_sent": 14,
+  "year": 2026,
+  "last_sent_at": "2026-04-09T14:23:55.586085+00:00"
+}