Let dual-role members choose their attendance role at RSVP and check-in

[mroderick]

Fixes #3040

Summary

A member subscribed to a chapter as both student and coach could only attend in one role. The invitation email fixed the role to whichever invitation pass ran first, and self check-in refused the other role. This change lets the member choose the role at RSVP and at the door.

It is a short-term fix: I'm working on a proposal to change the data model a bit, to make it adapt to codebar, and not the other way around.

What changes

• The invitation landing page shows both role buttons, "Attend as a student" and "Attend as a coach", to members subscribed as both. The chosen role wins and can change until the member accepts or joins a waiting list, and a cancelled member can choose again.
• Joining the waiting list takes the chosen role, so a dual-role member can join the student or the coach waiting list.
• Self check-in changes the role of an undecided invitation instead of refusing the mismatch.
• A role assignment requires a subscription to that role, on the workshop RSVP action, the invitation actions and self check-in. A member without subscriptions can still check in in the role of their invitation, and the event page keeps offering both roles to every member, as its UI always has.
• The role is written in the same update that completes the accept, the waiting-list join or the check-in, so a refused request changes nothing.
• Single-role members see no change in the normal flow.

Review notes

Focus on the role-flip rules in RoleChoiceConcerns and the check-in behaviour change.

Risks: an accepted or waitlisted invitation must keep its role. A flip on a waitlisted invitation would corrupt the per-role waiting list, because the waiting list reads the role from the invitation.

Deliberately not done, to keep this small: the invitation email copy still carries the provisional role, a member on a waiting list must leave it before changing role, and the "no spots left" message is unchanged. Those go with the groups-removal proposal.

Detail

The workshop page already offered "Attend as a student" and "Attend as a coach" to dual-role members, and WorkshopsController#rsvp already flipped the invitation role to the member's choice. This change extends the same semantics to the surfaces where the bug bit: the invitation landing page that the email links to, the waiting-list join, and self check-in. The subscription rule makes the server enforce exactly what each surface's views offer.

RoleChoiceConcerns holds the rule. The role can change while the invitation is undecided and the member is subscribed to the role they take. It stays fixed once the member accepts or once the invitation carries a waiting-list entry. The chosen role is evaluated against those rules and written with the action it completes.

The waiting-list partial now takes the role and the button label as locals, so the chooser can render one join form per role. The student and coach join forms merged into one, which is the only visible change to the existing full-role flow.

[mroderick]

The CI failure in Tests (Group 0) is a flake in the admin member search spec, unrelated to this PR. The search matches name and email with ILIKE '%Julie%', and a Faker draw containing "julie" (about 0.09% per member) creates a second match, so the controller renders the pick list instead of redirecting. Fix in #3044.