Backfill required RTDB fields and add migrate:backfill tooling

[MaxwellBo]

Summary

• Adds reusable RTDB field backfill tooling (scripts/lib/field-backfill.ts, scripts/backfill-missing-fields.ts, yarn migrate:backfill) with a required-optional-fields manifest sourced from DEFAULT_* objects.
• Makes previously optional fields required in TypeScript (conference, caucus speakerDuration / speakerUnit / queueIsPublic, resolution amendmentsArePublic / requiredMajority) and removes UI fallbacks that papered over missing data.
• Documents dry-run, apply, and emulator usage in AGENTS.md.

Production backfill (run before merge)

Dry-run against production (read-only, no credentials):

yarn migrate:backfill -- --dry-run

Latest dry-run: 3824 paths across 66505 committees (speakerDuration 1584, speakerUnit 1219, requiredMajority 567, conference 205, amendmentsArePublic 186, queueIsPublic 63).

Apply with a Firebase service account that has Realtime Database admin access:

GOOGLE_APPLICATION_CREDENTIALS=path/to/sa.json yarn migrate:backfill

Test plan

• yarn migrate:backfill -- --dry-run completes and reports expected field counts
• Apply backfill on production with service account credentials
• Re-run dry-run; expect 0 paths to write
• Smoke-test committee conference field, caucus speaker settings, resolution majority/amendments visibility in the app

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