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.

src/api/openapi.js

@@ -247,6 +247,7 @@
  *           type: integer
  *         number:
  *           type: string
+ *           description: Auto-allocated via nextNumber('delivery')
  *         type:
  *           type: string
  *           enum: [delivery]
@@ -254,6 +255,10 @@
  *           type: integer
  *           nullable: true
  *           description: manifest.entity_id
+ *         externalNumber:
+ *           type: string
+ *           nullable: true
+ *           description: User-supplied reference number
  *         date:
  *           type: string
  *           format: date-time