Restrict signups to shiftcoordinator role

app/controllers/application_controller.rb

app/controllers/assignments_controller.rb

app/controllers/candidates_controller.rb

app/helpers/application_helper.rb

app/javascript/controllers/session_controller.js

app/views/assignments/_user_avatar.html.erb

app/views/candidates/_user_avatar.html.erb

app/views/sessions/_session.html.erb

db/migrate/20241220201657_add_shiftcoordinator_to_users.rb

db/schema.rb