Fix bind_param type mismatches and integer validation

- TemplateModel.php: fix bind_param "ssssiii" -> "sssssii" (5 strings not 4)
- manage_workflows.php: fix bind_param 'ssiiii' -> 'ssiiiii' (4 int columns)
- download_attachment.php, delete_attachment.php, get_template.php: replace is_numeric()
  with strict int cast+equality check to reject floats and scientific notation
- manage_recurring.php: validate JSON input before accessing schedule_type key

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

api/manage_workflows.php

@@ -120,7 +120,7 @@ try {
             $stmt = $conn->prepare("UPDATE status_transitions SET
                                     from_status = ?, to_status = ?, requires_comment = ?, requires_admin = ?, is_active = ?
                                     WHERE transition_id = ?");
-            $stmt->bind_param('ssiiii',
+            $stmt->bind_param('ssiiiii',
                 $data['from_status'],
                 $data['to_status'],
                 $data['requires_comment'] ?? 0,