chore(release): v1.6.1-aio.18

- #62 `tickets_available` is always published, including zero. Omitting
  it used to mean "unlimited", which nothing else agreed with:
  api_get_event and api_ticket_create both read `amount_tickets < 1` as
  sold out, so a zero-capacity event advertised unlimited tickets on the
  card and returned 410 to every purchase. Three such events were live
  on aio-demo. The admin form no longer offers 0 either.

No schema change. Existing zero-capacity events are not migrated — we
cannot infer whether the organiser meant unlimited or forgot to set a
number — and they will read as sold out once republished. The #55 sweep
will not flag them on its own, so /republish-all is the way to refresh.

config.json

@@ -1,6 +1,6 @@
 {
   "id": "events",
-  "version": "1.6.1-aio.17",
+  "version": "1.6.1-aio.18",
   "name": "Events",
   "repo": "https://git.atitlan.io/aiolabs/events",
   "short_description": "Sell and register event tickets",