feat: add the product code to the travel export

docs/api-consumer-guide.md

@@ -172,6 +172,7 @@ an error: it returns `200` with an empty `data` array and the usual `meta`.
     {
       "busProId": 12794,
       "code": "PCJSER261226",
+      "productCode": "PCJSER",
       "product": "Semesterende - Portes du Soleil - Joliment",
       "dateFrom": "2026-12-26",
       "dateTo": "2026-12-30",
@@ -212,6 +213,11 @@ an error: it returns `200` with an empty `data` array and the usual `meta`.
 is always an int and never null; `hotel` is always an object and never null; `pickups` is always an array, possibly
 empty, never null.
 
+**`productCode`** is the code of the travel (product) the date belongs to, e.g. `DKIWE`. Key on it to
+recognise dates of the same product. Do not derive it from the date `code` by cutting off the date: some
+products run dates under a different code (`DWWMP` runs `DWWMW…`), and group travels carry each group's own
+code on the date. It is `null` for the few travels that have no code upstream.
+
 **`meta` describes the whole export, not the page.** `total`, `totalPages`, `skipped`, `pickupsWithoutMasterData`,
 `sourceGeneratedAt` and `generatedAt` are identical on every page, so you can read them once on page 1 and decide whether to walk the
 rest. `total` is the row count **after** skips, and `totalPages` is `ceil(total / perPage)`.