Headless refresh-token -> access-token exchange via OIDC discovery + token
endpoint. Cached to expiry (60s margin), thread-safe, persists rotated refresh
tokens, graceful null on invalid_grant/network errors. Wired into DI when
online_inbox is enabled. Interactive PKCE login (UI) still pending the
registered redirect URI. 7 tests, stubbed HttpMessageHandler.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>