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/update.md

@@ -4,8 +4,10 @@ Protected. Requires a valid session cookie (`credentials: 'include'`).
 
 Update delivery
 
-Patches header fields that are present. If `lines` is present, replaces
-all lines; if omitted, leaves lines unchanged. Returns the full delivery.
+Patches header fields that are present. The manifest `number` is
+auto-allocated and cannot be patched; only the user-supplied
+`externalNumber` reference is editable. If `lines` is present,
+replaces all lines; if omitted, leaves lines unchanged.
 
 
 ## Path parameters
@@ -18,7 +20,7 @@ all lines; if omitted, leaves lines unchanged. Returns the full delivery.
 
 ```json
 {
-  "number": "DHL-124",
+  "externalNumber": "DHL-124",
   "lines": []
 }
 ```
@@ -34,6 +36,7 @@ Updated delivery
     "number": "…",
     "type": "delivery",
     "supplierId": 1,
+    "externalNumber": "…",
     "date": "2026-08-10T00:00:00.000Z",
     "lines": [
       {