fix(newsletter): use follow_redirects=False to avoid false positive on 404 redirect

Beehiiv redirects non-existent newsletters to /?404=... with HTTP 302.
With follow_redirects=True, the final 200 was misread as "newsletter exists".
Fix: disable redirect following so only a direct HTTP 200 = newsletter real.
Also reset state back to last_sent=13 (real).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cron/newsletter-cercetasi-state.json

@@ -1,5 +1 @@
-{
-  "last_sent": 13,
-  "year": 2026,
-  "last_sent_at": "2026-04-02T18:59:37.878273+00:00"
-}
+{"last_sent": 13, "year": 2026, "last_sent_at": "2026-04-02T18:18:46.000000+00:00"}