New TurnKey Mibew version (19.0)

Changes:

• Rebuild on Debian 13 (Trixie) with PHP 8.4 and MariaDB 11.8.
• Install Mibew 3.6.0 from its official release archive and verify the upstream-published SHA-256 digest.
• Update the database bootstrap for current MariaDB and retain Mibew's supervised upstream update path.
• Store ungrouped support threads as SQL NULL so MariaDB can enforce Mibew's declared group foreign key.

Links

• Release meta-files (signature, manifest)