Redirect instead of a 500 when the waiting-list entry is already gone

[mroderick]

Removing a waiting-list entry that no longer exists rendered a 500. This PR guards the lookup in WaitingListsController#destroy and redirects with a notice instead.

One of five small PRs from the same pre-existing-gap audit against this flow. Heads are reviewed independently; if a sibling merges first expect a trivial rebase, the branches touch different regions of the same spec files.

Detail

WaitingListsController#destroy calls .destroy on WaitingList.find_by(invitation_id: @invitation.id), which is nil when the entry is already gone — reachable by replaying a "Remove from the waiting list" link (double-click, or a bookmarked token URL after the entry was consumed by a promotion). A consumed entry is more likely after the promotion fix lands, since promotions destroy the next entry in the reject flow.

Now: no entry means redirect to the invitation page with notice "You are not on the waiting list", and no waiting_list.left activity recorded.

Review notes

• The notice text is a literal, matching this controller's existing style ("You have been removed from the waiting list").
• Param name is invitation_id, which carries the invitation token: the token is still the only credential, unchanged.

Sibling PRs:

• Rejecting drops a cancelling member's waiting-list entry #2997 (fix/waiting-list-self-promotion-on-reject)
• 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)