Re-enable Turbo Drive on Recipients Shout-Out Form

[1steve78]

Re-enable Turbo Drive on recipients shout-out form and ensure 303 redirect

Re-enables Turbo Drive on the recipients page shout-out form and aligns controller guard redirects with HTTP 303 See Other so submissions feel faster and avoid unnecessary full-page reloads.

What is the goal of this PR and why is this important?

• Re-enable native Turbo Drive on the recipients shout-out form (app/views/events/recipients.html.erb) as part of the audit of remaining turbo: false occurrences following [PR #2619](Re-enable Turbo Drive on admin forms and replace redundant submit-onc… #2619).
• Prevent duplicate submissions by disabling the submit button while the request is in flight.
• Ensure controller guard redirects comply with Hotwire Turbo standards using HTTP 303 See Other.
• Track this work under issue Audit and safely remove unnecessary turbo: false from admin forms #2635.

How did you approach the change?

• Controller status codes: Added status: :see_other (303) to the guard redirect in EventsController#feature_recipient_shoutout when the recipient selection is blank or locked.
• Form opt-out removal: Removed data: { turbo: false } from the shout-out form_with block.
• Submit-in-progress state: Added data-turbo-submits-with="Adding…" to the submit button.
• Request specs: Updated spec/requests/events_spec.rb to assert have_http_status(:see_other) across the relevant redirect pathways.
• System specs: Added spec/system/events_recipient_shoutout_spec.rb to verify Turbo submission, TomSelect selection, alert rendering, and button disabling.

UI testing checklist

• Visit an event's recipients page (/events/:id/recipients) as an admin.
• Select a scholarship recipient from the dropdown and click Add shoutout. Verify the button displays “Adding…” and the request redirects to the registration edit form.
• Confirm the registration's shout-out toggle is enabled and any seeded bio appears.
• Return to the recipients page and submit without selecting a recipient. Verify the warning alert, “Choose a recipient to feature.”, displays correctly under Turbo Drive.

Anything else to add?

• searchable_select_controller.js already implements Turbo cache and disconnect() teardown handlers, so no JavaScript changes were required.
• This is the first PR in the incremental series to safely re-enable Turbo Drive across the application.