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/get_template.php

@@ -24,18 +24,15 @@ try {
     }
 
     // Get template ID from query parameter
-    $templateId = $_GET['template_id'] ?? null;
+    $templateId = isset($_GET['template_id']) ? (int)$_GET['template_id'] : 0;
 
-    if (!$templateId || !is_numeric($templateId)) {
+    if ($templateId <= 0 || (string)$templateId !== (string)($_GET['template_id'] ?? '')) {
         ErrorHandler::sendValidationError(
             ['template_id' => 'Valid template ID required'],
             'Invalid request'
         );
     }
 
-    // Cast to integer for safety
-    $templateId = (int)$templateId;
-
     // Get template
     $conn = Database::getConnection();
     $templateModel = new TemplateModel($conn);