Persist deputy selection from bias proposal edits

The bias proposal edit UI accepts a deputy selection but ProposalsController#update silently discards it. Evidence from local meeting 5, proposal 5 for Mikke Mus: requests submitted proposal[biased_deputy_role_id]=38 and later =39, while Bias::Invitation#replacement_role_id and Bias::Declaration#deputy_role_id remained nil. The update path excludes biased_deputy_role_id from permitted_params and setup_bias_records! reads proposal.biased_deputy_role (the existing declaration value) instead of the submitted role. Persist and validate the submitted deputy atomically through the canonical bias lifecycle, and surface a validation error instead of silently losing the selection.