Inbox private realtime delivery hotfix

This hotfix restores live /inbox chat updates after the scoped broadcast hardening release.

• Publishes server-side webhook broadcasts with private: true, matching the app's private workspace Realtime subscriptions.
• Moves broadcast.self into the Supabase channel config object so SDK self-delivery semantics are actually enabled.
• Adds low-noise postgres_changes safety-net error/timeout warnings without logging customer PII.