chore(release): v1.6.8-aio.2

Two wave fixes found while checking whether the webapp had been updated
for v1.6.8.

- #65 keep ticket waves when a client edits an event without them. A
  client that rebuilt the `extra` envelope rather than round-tripping it
  destroyed every wave: the list arrived empty, a single primary wave was
  synthesized from the event-level `amount_tickets`, and a multi-wave
  event silently collapsed into one tier. `promo_codes` had carried the
  same guard since the v1.6.8 merge; `ticket_waves` is the same class of
  organiser state and now carries it too.
- #66 expose `ticket_waves` on public event responses. A buyer cannot
  choose a tier without its id, and nothing public carried one — the
  NIP-52 tags describe the active wave but name no id. Waves move to
  `EventExtraBase`, leaving promo codes as the only organiser-private
  field in `extra`. Buyers can now see upcoming tiers and their prices,
  which is what #61 recorded as the cost of the flat-tag decision.

No schema change; both are model/serialisation only.

Verified against bohm's dev LNbits before tagging: the exact PUT that
collapsed a two-wave event now leaves both intact with the roll-up
unchanged, the public endpoint carries the waves while still hiding
promo codes, and a webapp-shaped edit that writes through to the primary
wave takes effect (139 = 99 + 40) where it was previously discarded.

#66 is a prerequisite for aiolabs/webapp#176, which is merged to dev and
needs this deployed before its wave picker works for anyone but the
organiser.

config.json

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