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>

api/manage_recurring.php

@@ -147,18 +147,21 @@ function calculateNextRun($scheduleType, $scheduleDay, $scheduleTime) {
             break;
 
         case 'weekly':
-            $days = ['', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];
-            $dayName = $days[$scheduleDay] ?? 'Monday';
+            $days = [1 => 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];
+            $dayName = $days[(int)$scheduleDay] ?? 'Monday';
             $next = new DateTime("next {$dayName} " . $time);
             break;
 
         case 'monthly':
-            $day = max(1, min(28, (int)$scheduleDay));
+            $day = max(1, min(31, (int)$scheduleDay));
             $next = new DateTime();
             $next->modify('first day of next month');
-            $next->setDate($next->format('Y'), $next->format('m'), $day);
-            list($h, $m) = explode(':', $time);
-            $next->setTime((int)$h, (int)$m, 0);
+            // Clamp to last day of target month (handles Feb, 30-day months)
+            $daysInMonth = (int)$next->format('t');
+            $day = min($day, $daysInMonth);
+            $next->setDate((int)$next->format('Y'), (int)$next->format('m'), $day);
+            $parts = explode(':', $time . ':00'); // ensure at least H:M
+            $next->setTime((int)$parts[0], (int)$parts[1], 0);
             break;
 
         default: