ver. 0.321: API produkty — obsługa custom_fields w create/update

- ProductsApiController: parsowanie custom_fields z body (name, type, is_required)
- Zaktualizowano docs/API.md

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

docs/API.md

@@ -302,11 +302,15 @@ Content-Type: application/json
     }
   },
   "categories": [1, 5],
-  "products_related": [10, 20]
+  "products_related": [10, 20],
+  "custom_fields": [
+    {"name": "Napis na koszulce", "type": "text", "is_required": 1}
+  ]
 }
 ```
 
 Wymagane: `languages` (min. 1 jezyk z `name`) oraz `price_brutto`.
+`custom_fields` — opcjonalne; kazdy element wymaga `name`, `type` (domyslnie `text`), `is_required` (0/1).
 
 Odpowiedz (HTTP 201):
 ```json