style: declare(strict_types=1) on PSA_Upgrade to match project pattern

QA caught this on 37aa0c8 — every other includes/class-psa-*.php in the
codebase declares strict_types at the top. The new class should match.

Agent-Session: cowork-qa-p1-fixes-20260414

includes/class-psa-upgrade.php

@@ -16,6 +16,8 @@
  * @package PeptideSearchAI
  */
 
+declare( strict_types=1 );
+
 if ( ! defined( 'ABSPATH' ) ) {
 	exit;
 }