Per-session supervisor for a TDLib account.

Supervises Sexy.TDL.Backend and Sexy.TDL.Handler with a :one_for_all strategy (if one crashes, both restart). Additional child specs can be injected via the :children option in Sexy.TDL.open/3.

Started automatically by Sexy.TDL via DynamicSupervisor — not called directly.

Per-session resilience lives here: :one_for_all restarts the Backend/Handler pair up to 5 times in 30 seconds (e.g. when the tdlib binary dies). A session that keeps failing dies alone — restart: :temporary ensures it is never resurrected by the AccountVisor, so one broken account can't cascade into the others. Monitor the pid returned by Sexy.TDL.open/3 to detect this.