Prevent failed bias proposal creation from leaving stale speakers

Meeting 14 Chrome acceptance attempted to create a bias proposal for C14 with D09 and a valid legal reason. The proposal did not persist, but a transient Habilitet speaker appeared. Starting it requested `/meetings/14/secretary/ongoing_case/38/speakers/149/start` and raised ActiveRecord::RecordNotFound; reload removed the stale speaker. Make formal bias registration, generated speaker creation, declaration/invitation setup, and canonical command acceptance atomic. On failure render validation in the modal and broadcast no proposal/speaker artifacts. Add rollback coverage for candidate rejection, command conflict, and validation failure.