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_recurring.php

@@ -70,6 +70,10 @@ try {
                 echo json_encode($result);
             } else {
                 $data = json_decode(file_get_contents('php://input'), true);
+                if (!is_array($data) || empty($data['schedule_type']) || empty($data['title_template'])) {
+                    echo json_encode(['success' => false, 'error' => 'schedule_type and title_template are required']);
+                    exit;
+                }
 
                 // Calculate next run time
                 $nextRun = calculateNextRun(
@@ -94,6 +98,10 @@ try {
             }
 
             $data = json_decode(file_get_contents('php://input'), true);
+            if (!is_array($data) || empty($data['schedule_type'])) {
+                echo json_encode(['success' => false, 'error' => 'Invalid request data']);
+                exit;
+            }
 
             // Recalculate next run time if schedule changed
             $nextRun = calculateNextRun(