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>

src/newsletter_cercetasi.py

@@ -35,10 +35,14 @@ def _write_state(state: dict) -> None:
 
 
 async def _newsletter_exists(n: int, year: int) -> bool:
-    """Return True if newsletter #{n}/{year} returns HTTP 200."""
+    """Return True if newsletter #{n}/{year} exists (HTTP 200, no redirect to 404 page).
+
+    Beehiiv redirects non-existent newsletters: /p/newsletter-N-din-YEAR → /?404=... (302)
+    A real newsletter returns 200 directly without redirect.
+    """
     url = NEWSLETTER_BASE_URL.format(n=n, year=year)
     try:
-        async with httpx.AsyncClient(follow_redirects=True) as client:
+        async with httpx.AsyncClient(follow_redirects=False) as client:
             resp = await client.get(url, timeout=10)
             return resp.status_code == 200
     except Exception as e: