Add phpDoc typehints for magic methods

Signed-off-by: Raul <raul@nextcloud.com>

lib/Db/Card.php

@@ -158,6 +158,17 @@ class Card extends RelationalEntity {
 		return $calendar;
 	}
 
+	public function getDaysUntilDue(): int {
+		$today = new DateTime();
+		$today->setTime(0, 0);
+
+		$match_date = $this->getDueDateTime() ?? new DateTime();
+		$match_date->setTime(0, 0);
+
+		$diff = $today->diff($match_date);
+		return (int) $diff->format('%R%a'); // Extract days count in interval
+	}
+
 	public function getCalendarPrefix(): string {
 		return 'card';
 	}