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/get.md

@@ -23,6 +23,7 @@ Delivery
     "number": "…",
     "type": "delivery",
     "supplierId": 1,
+    "externalNumber": "…",
     "date": "2026-08-10T00:00:00.000Z",
     "lines": [
       {