refactor(launch): collapse helcim-pay duplication and use setAuthCookie helper

Follow-up to 51230e5. /simplify review surfaced residual duplication
and a timer leak.

- useHelcimPay: extract _initializeTicket(metadata, errorPrefix) to
  collapse initializeTicketPayment + initializeSeriesTicketPayment
  (95% identical bodies). Drop the dead `amount` arg from initialize-
  TicketPayment — server re-derives ticket amounts in initialize-
  payment.post.js and never reads body.amount for ticket types.
  Capture timer ids and clearTimeout on resolve/reject so the 10-min
  payment timer and 5-second observer timer stop leaking after every
  payment.
- EventTicketPurchase: caller updated for the dropped arg.
- verify.post.js: replace inline jwt.sign + setCookie block with the
  setAuthCookie(event, member) helper. verify was the last hand-rolled
  caller after the helper was extracted in 208638e.
- LAUNCH_READINESS: add simplify-pass-followups bullet pointing to the
  six deferred items in docs/TODO.md.

Tests: 758 passing, 2 skipped, 0 failing.

app/components/EventTicketPurchase.vue

@@ -330,7 +330,6 @@ const handleSubmit = async () => {
       await initializeTicketPayment(
         props.eventId,
         form.value.email,
-        ticketInfo.value.price,
         props.eventTitle,
       );