chore(psalm): Fix annotations to make psalm happy

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/Db/SessionMapper.php

@@ -29,6 +29,7 @@ use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Db\QBMapper;
 use OCP\IDBConnection;
 
+/** @template-extends QBMapper<Session> */
 class SessionMapper extends QBMapper {
 	public function __construct(IDBConnection $db) {
 		parent::__construct($db, 'deck_sessions', Session::class);