Replace legacy bias proposal candidate references

In progress. General secretary/politician proposal-form creation and participant-screen formal bias creation now use one meeting-locked registrar: candidate selection/validation uses canonical role IDs, declaration intent is persisted with the proposal, pending overlay/receipt/outbox registration is atomic, and failures roll back proposal/invitation/declaration state. Routed flows suppress the legacy Proposal voting-right callback. Remaining before completion: move case-start proposal generation into the canonical case lifecycle coordinator, then remove Proposal#biased_deputy, the legacy callback compatibility path, and the physical legacy candidate column/runtime references.