feat: API produktów - nowe pola new_to_date i additional_message (v0.332)

- ProductRepository::getProductForApi() eksportuje new_to_date, additional_message,
  additional_message_required, additional_message_text
- ProductsApiController obsługuje te pola w PUT/PATCH
- Zaktualizowana dokumentacja API.md i CHANGELOG

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

docs/API.md

@@ -216,6 +216,10 @@ Odpowiedz:
     "weight": 0.5,
     "stock_0_buy": 0,
     "custom_label_0": null,
+    "new_to_date": null,
+    "additional_message": 0,
+    "additional_message_required": 0,
+    "additional_message_text": null,
     "set_id": null,
     "product_unit_id": 1,
     "producer_id": 3,
@@ -294,6 +298,10 @@ Content-Type: application/json
   "status": 1,
   "sku": "PROD-001",
   "ean": "5901234123457",
+  "new_to_date": null,
+  "additional_message": 0,
+  "additional_message_required": 0,
+  "additional_message_text": null,
   "weight": 0.5,
   "languages": {
     "pl": {
@@ -330,6 +338,10 @@ Content-Type: application/json
 {
   "price_brutto": 129.99,
   "status": 1,
+  "new_to_date": "2026-12-31",
+  "additional_message": 1,
+  "additional_message_required": 0,
+  "additional_message_text": "Dodaj tresc do personalizacji",
   "languages": {
     "pl": {
       "name": "Zaktualizowana nazwa"
@@ -339,6 +351,7 @@ Content-Type: application/json
 ```
 
 Partial update — wystarczy przeslac tylko zmienione pola. Pola nieprzeslane zachowuja aktualna wartosc.
+Pola ustawien produktu obslugiwane przez API: `new_to_date`, `additional_message`, `additional_message_required`, `additional_message_text`.
 
 Odpowiedz: pelne dane produktu (jak w `get`).