Add explicit per-call voice history controls

[JulioMCruz]

Summary

• default completed-turn Chat mirroring on for new calls
• add visible private/off opt-out with explicit status
• bind direct/project calls to their canonical conversation scope
• send an explicit fail-safe none policy when mirroring is off

Validation

• 355/355 tests
• typecheck
• production build (51 routes)
• changed-file lint and secret scan