Fix recurring ticket schedule edge cases in API and model

- manage_recurring.php calculateNextRun(): expand monthly cap from 28→31
  with proper last-day-of-month clamping (matches model fix); use split
  with ':00' append to handle malformed time strings without crashing;
  fix weekly day array to start at index 1 (not 0) so day=0 never maps
  to empty string and blows up DateTime
- RecurringTicketModel::calculateNextRunTime(): same weekly day array fix
  (start at index 1) to eliminate '' → DateTime exception on day=0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models/RecurringTicketModel.php

@@ -176,7 +176,8 @@ class RecurringTicketModel {
                 break;
 
             case 'weekly':
-                $dayName = ['', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'][$scheduleDay] ?? 'Monday';
+                $dayNames = [1 => 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];
+                $dayName = $dayNames[(int)$scheduleDay] ?? 'Monday';
                 $next = new DateTime("next {$dayName} " . $scheduleTime);
                 break;