monitoring: accept any HTTP status in TLS probe

Only care about TLS handshake success for certificate monitoring.
Services like nzbget (401) and ha (400) return non-2xx but have
valid certificates.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/monitoring/blackbox.nix

@@ -39,6 +39,7 @@ in
           http:
             fail_if_not_ssl: true
             preferred_ip_protocol: ip4
+            valid_status_codes: []  # Accept any status, we only care about TLS
     '';
   };