chore(release): v1.6.8-aio.3

- #67 honour per-wave fiat when the organiser set an explicit rail list.
  `effective_payment_methods` returned `extra.payment_methods` before
  consulting the wave, so the `wave` argument added for #61 did nothing
  in the common case — the webapp always sets that list. The NIP-52 tag
  advertised `tickets_payment_methods: lightning,fiat` while omitting
  `tickets_allow_fiat`, and the checkout offered a card button that
  `api_ticket_create` then refused. Reported from aio-demo.

Eight lines in `models.py`; no schema change, no migration,
`migrations.py` still byte-identical to upstream v1.6.8.

The organiser-facing symptom is already gone on demo — aiolabs/webapp#179
makes the Card checkbox govern every wave, and re-saving the event
corrected all four. This is the backend half: it stops the published tag
contradicting itself when a wave genuinely cannot take fiat, which the
LNbits admin's per-wave toggles still allow.

config.json

@@ -1,6 +1,6 @@
 {
   "id": "events",
-  "version": "1.6.8-aio.2",
+  "version": "1.6.8-aio.3",
   "name": "Events",
   "repo": "https://git.atitlan.io/aiolabs/events",
   "short_description": "Sell and register event tickets",