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/create.md

@@ -5,15 +5,18 @@ Protected. Requires a valid session cookie (`credentials: 'include'`).
 Create delivery
 
 Creates a goods receipt with header and lines in one transaction.
-Each line requires `sku`, `amount`, `net`, and `tax` (rate).
-`rebate` defaults to `0`; `transactedAmount` defaults to `amount`.
+The manifest `number` is allocated automatically via
+`nextNumber('delivery')`; the user-supplied reference is stored as
+`externalNumber`. Each line requires `sku`, `amount`, `net`, and
+`tax` (rate). `rebate` defaults to `0`; `transactedAmount` defaults
+to `amount`.
 
 
 ## Body (JSON)
 
 ```json
 {
-  "number": "DHL-123",
+  "externalNumber": "DHL-123",
   "date": "2026-08-10",
   "supplierId": 1,
   "lines": [
@@ -38,6 +41,7 @@ Created delivery
     "number": "…",
     "type": "delivery",
     "supplierId": 1,
+    "externalNumber": "…",
     "date": "2026-08-10T00:00:00.000Z",
     "lines": [
       {