AO3-4930 Remove Invited collection participant role

[pmonfort]

Pull Request Checklist

• Have you read the contributing guidelines?
• Have you read "How to write the perfect pull request"?
• Have you added tests for any changed functionality?
• Have you added the [Jira](https://otwarchive.atlass as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait until they are reviewed and merged before creating new pull requests.
• I acknowledge that submitting code created or modified with the help of AI is a bannable offense.

Issue

https://otwarchive.atlassian.net/browse/AO3-4930

Purpose

Removes the "Invited" collection participant role, which had no real purpose.

The only code path that acted on it was CollectionParticipantsController#join a participant with the Invited role who clicked "Join" was promoted to Member. In practice this almost never happened, because nothing in the codebase ever assigns the Invited role, adding someone from the participants page already creates them directly as a Member. The only way to reach the flow was for a maintainer to add a user and then manually change their role to "Invited" in the dropdown.

Changes:

• CollectionParticipant: remove the INVITED constant, the "Invited" option from the role dropdown, and is_invited?.
• Collection#invited and User#invited_collections: re
• CollectionParticipantsController#join: remove the branch that accepted invitations. Joining a collection you already participate in now always shows "You have already joined (or applied to) this collection."
• Add the After:remove_invited_collection_participant_role task to convert any existing Invited rows to None ("applied to join").
• Remove the "preapprove a user" Cucumber scenario and the controller spec for the invited path; add specs for the after task and for "Invited" no longer being a valid role.

After Deploy Steps

1. Run the after task before deploying the code:

   bundle exec rake After:remove_invited_collection_parti

It converts any remaining Invited collection participants to None and prints the number of rows updated. It is idempotent and safe to re-run.

2. Ordering matters: once the new code is live, Invited is no longer an allowed participant_role. Any leftover row would fail validation ("That is not a valid participant role.") when a maintainer edits that participant, and the role dropdown on the participants page would have no selected option for them.

3. No schema change, no index change, no reindex needed. Safe to include in a regular release.

4. Rollback: revert the code only. Converted rows stay as None, which is a valid role in both versions.

Testing Instructions

1. As a collection owner, go to the collection's Participants page.
2. Check the role dropdown for any participant: it should list None, Member, Moderator and Owner only.
3. Add a new participant via the "participants_to_invite" field: they should be added as a Member, as before.
4. As a user who has already applied to (or is a member of) a moderated collection, press "Join" on the collection page: you should see "You have already joined (or applied to) this collection."
5. As a user who is not yet a participant, press "Join": you should see "You have applied to join …", as before.

Credit

Pablo Monfort (he/him)