feat: let requesters observe shared agent turns

[akkikumar72]

Summary

• add an opt-in requester observer policy while keeping owner-only as the default
• encrypt turn-scoped observer activity to the actual requesters without exposing owner-only controls, configuration, or management events
• let desktop clients render requester-addressed activity only while an authenticated requester turn is active

Security model

• derive requester recipients from triggering event authors, including multi-author and cancelled batches
• keep requester metadata internal to the harness and out of serialized observer payloads
• preserve per-recipient pacing and owner-wide visibility
• keep stop controls and other privileged side effects owner-only

UI

With requester visibility enabled, the requester can follow live output for their own active turn while owner-only stop controls remain disabled.

Validation

• just ci
• cargo test -p buzz-acp
• cargo test -p buzz-relay --lib against a clean isolated database
• pnpm build:e2e
• focused desktop Playwright requester-visibility test

Fixes #2716