Add booked invoice helper and retrieval methods

- Introduce `economic_invoice_booked` helper for normalizing booked invoice data.
- Add `getFromId` method to fetch booked invoice details by ID.
- Extend `economic` class with `getInvoiceBookedFromExternalId` for external ID-based retrieval.
- Update imports and economic helper class registrations to include booked invoice functionality.

services/nginx/app/classes/economic.php

@@ -23,6 +23,7 @@ use endpoints\economic_payment_terms_endpoint;
 use endpoints\economic_products_endpoint;
 use helpers\economic_customer;
 use helpers\economic_invoice_draft;
+use helpers\economic_invoice_booked;
 use helpers\economic_tasks;
 use interfaces\economic_i;
 
@@ -141,4 +142,15 @@ class economic implements economic_i
 
         return new $this->helpers->economic_customer($customer_number);
     }
+
+    /**
+     * @throws \Exception
+     */
+    public function getInvoiceBookedFromExternalId(string $external_id): economic_invoice_booked
+    {
+        $bookedId = $this->invoices->booked->get_from_external_id($external_id);
+        $raw = $this->invoices->booked->getFromId($bookedId);
+        // Return the booked invoice helper
+        return (new economic_invoice_booked($raw));
+    }
 }