Notification For Upcoming Certificate Expiry

**Is your feature request related to a problem? Please describe.**

Checks don't notify of certificate expiry until after the certificate has expired.

**Describe the solution you'd like**

Notifications that a TLS certificate is going to expire in 14; 7, 1 day etc.

**Describe alternatives you've considered**

N/A

**Additional context**

UptimeKuma does it at 21, 14, 7, 1 days from memory.

Start by locating how Checkmate detects TLS certificate expiry and delivers check notifications. Confirm the warning intervals to support, using the issue’s 14-, 7-, and 1-day examples, then verify each warning occurs before expiry without replacing the existing post-expiry notification.

Written by the indexing model from the issue text.