- session_manager/manager.rs: remove slog::Logger field, update new()
and with_config() to remove log parameter
- email/lib.rs: remove slog::Logger from AppEmail::init()
- rpc/admin/server.rs: remove slog::Logger from serve() and spawn(),
replace with tracing::info!/error!
Converts AppEmail from blocking sync sends to a background worker via
mpsc channel, adds SMTP pool tuning (min_idle 5, max_size 100), and
3-retry backoff on send failures.
