fix(messages): isolate broadcast history by recipient roster

[rissrice2105-agent]

Summary

• reuse broadcast conversations only when the resolved participant roster matches exactly
• preserve the original participant access boundary when applicants or status filters change
• add regression coverage for both matching and changed rosters

Fixes #529

Validation

• pnpm exec vitest run src/app/api/messages/broadcast/route.test.ts (16/16)
• pnpm exec tsc --noEmit
• pnpm exec eslint src/app/api/messages/broadcast/route.ts src/app/api/messages/broadcast/route.test.ts
• git diff --check