Fix DoubleRenderError on unsubscribe link for members with incomplete details - #2836

Problem

Rollbar items 660 and 559 report AbstractController::DoubleRenderError on GET /unsubscribe/:token.

MembersController#unsubscribe sets the member's session, then calls authenticate_member! and afterwards unconditionally redirects to subscriptions_path. For members whose profile is incomplete, finish_registration (inside authenticate_member!) already redirects to edit_member_details_path, so the second redirect raised DoubleRenderError. The cascade then bounced twice more: the inline rescue StandardError caught it and called redirect_to root_path on the already-performed response (#559), and the production rescue_from Exception handler failed to render the already-committed response (#660). One click, three chained render errors.

This only affects members who click an unsubscribe email link before completing registration details. Every other authenticate_member! caller runs it as a before_action, where Rails halts the chain on redirect — this was the only explicit call site.

Change

• Guard the second redirect in unsubscribe with unless performed? (app/controllers/members_controller.rb:40). Members with incomplete details now land on the details page (the intended redirect); everyone else still reaches their subscriptions page. With the first conflict gone, neither the rescue arm nor the error handler can fire on a performed response, so both Rollbar items are fixed.
• Add a controller spec for the incomplete-details path (spec/controllers/members_controller_spec.rb). The spec fails with the exact production error when the guard is reverted.

Verification

• bundle exec rspec spec/controllers/members_controller_spec.rb — 3 examples, 0 failures
• bundle exec rubocop on both changed files — no offenses
• Mutation check: reverting the guard reproduces the production DoubleRenderError in the new spec