Delivery API: use externalNumber for user reference

Delivery create now accepts externalNumber (user-supplied reference) instead
of number; the manifest number is auto-allocated server-side via
nextNumber('delivery'). Update patches externalNumber (clearable) rather than
number. DeliverySummary schema documents externalNumber and notes number is
auto-generated. Regenerate API docs.

docs/api/routes/delivery/list.md

@@ -20,6 +20,7 @@ Delivery list
       "number": "…",
       "type": "delivery",
       "supplierId": 1,
+      "externalNumber": "…",
       "date": "2026-08-10T00:00:00.000Z"
     }
   ]