Improved Host Status Notifications (up to 10k hostnames in one email)

Big update for RedirHub emails! Now handling thousands of hosts, with CSV exports and automatic hostname change notifications for everyone.

Emails now handle thousands of hosts (previously limited to ~50).

Each email shows up to 20 hostnames for readability.

For larger batches, a CSV file with all hosts is attached.

All users are now subscribed by default to hostname change notifications, since the new system avoids the old issue of too-frequent emails.