Rejecting drops a cancelling member's waiting-list entry

[mroderick]

Rejecting an RSVP while the member is themselves waitlisted can hand the seat back to the person who just cancelled. This PR removes the cancelling member's waiting-list entry during reject, so the promotion goes to the real next entry for the role.

One of five small PRs from the same pre-existing-gap audit against this flow (cross-role promotion pin, promotion email pin, remove-of-consumed-entry guard, token-only reject). Heads are reviewed independently; if a sibling merges first expect a trivial rebase, the branches touch different regions of the same spec file.

Detail

WorkshopInvitationController#reject sets attending: false but leaves the member's WaitingList row in place. Two consequences:

• The member keeps an entry that can later auto-promote them onto a seat they declined.
• Worse: with the cancelling member's own entry first on the list, the promotion lookup
  (WaitingList.next_spot(workshop, role)) returns it. The flow then destroys that entry, re-sets the
  freshly cancelled invitation to attending: true, and emails the member they are attending again —
  the self-promotion cascade.

Fix: destroy the cancelling member's entry (WaitingList.find_by(...)&.destroy) before computing the next spot, so a rejection cannot list the member back in.

Review notes

• Ordering matters: the entry must be destroyed before next_spot is computed; the third new spec (member behind is promoted, canceler not) exercises exactly that.
• attending: false members can also hold waitlist entries (a member who joined the list then cancelled), so the lookup uses find_by without an attending filter, same as the existing delete flow.
• Deliberately out of scope: the closed-waitlist view guard, the destroy gate matrix, admin removal between freeze and start, promote_next update-failure path, concurrency, and the promotion/freeze boundary — all owned by feat/post-close-rsvp-waitlist (which renames next_spot to promote_next; the rule above carries over).

Sibling PRs:

• Redirect instead of a 500 when the waiting-list entry is already gone #2998 (fix/waiting-list-destroy-missing-entry)
• Skip CSRF for reject (token-authenticated, no session) #2999 (fix/reject-token-no-session)
• Pin cross-role waiting list promotion #3000 (test/waitlist-cross-role-promotion)
• Assert the waiting-list promotion email on reject #3001 (test/waitlist-promotion-email)