fix(machine): a taken endpoint id is a conflict, not a server fault

A host reporting an endpoint id another machine already holds hit the unique
index, and the raw refusal reached the global handler as a 500 -- telling a
host its beat broke the server rather than that the id is taken. It is now the
409 every other conflict here gives, and the route documents it.

Checked-then-written would be worse rather than better: two hosts reporting the
same id in the same instant both read "nobody holds it" and both write, which
is precisely what the index is for. The read would add a query and remove
nothing.

Before:
  expect(res.status).toBe(409)
  Received: 500

apps/api/app/routes/machine.ts

@@ -239,7 +239,8 @@ export namespace MachineApi {
 					},
 					400: ErrorResponses[400],
 					403: ErrorResponses[403],
-					404: ErrorResponses[404]
+					404: ErrorResponses[404],
+					409: ErrorResponses[409]
 				}
 			}),
 			validator(