Complete voting engine integration

Ship the Rust-backed voting replacement with agreed current-production feature parity and minimal safe deployment plumbing. Rust owns new voting runs; Rails retains authorization, voting-right source data, encrypted content and the surrounding UI. Keep the work deliberately bounded: restore actively used parity, preserve historical access with the simplest safe read-only or migration approach, prevent lifecycle regressions, configure database/secrets/connectivity/backups, validate in staging, and cut over. Do not turn this into a broader voting redesign, observability program, distributed-systems framework or unrelated Rails-to-Rust migration.