feat: Add variation-level license duration to product editor

- Added license_duration_days field to ProductVariant type
- Added License Duration input to each variation card
- Backend: ProductsController saves/loads variation-level _license_duration_days meta
- Allows different license periods per variation (e.g., 1-year, 2-year, lifetime)

includes/Api/ProductsController.php

@@ -898,6 +898,7 @@ class ProductsController {
 					'image_id' => $variation->get_image_id(),
 					'image_url' => $image_url,
 					'image' => $image_url, // For form compatibility
+					'license_duration_days' => get_post_meta($variation->get_id(), '_license_duration_days', true) ?: '',
 				];
 			}
 		}
@@ -994,6 +995,11 @@ class ProductsController {
 			$saved_id = $variation->save();
 			$variations_to_keep[] = $saved_id;
 			
+			// Save variation-level license duration
+			if (isset($var_data['license_duration_days'])) {
+				update_post_meta($saved_id, '_license_duration_days', self::sanitize_number($var_data['license_duration_days']));
+			}
+			
 			// Manually save attributes using direct database insert
 			if (!empty($wc_attributes)) {
 				global $wpdb;