Refuse to start decnet.web.api when DECNET_MODE=agent (unless the
operator explicitly opts into dual-role with DECNET_DISALLOW_MASTER=
false). The Typer CLI already hides master-only commands on agents,
but a misconfigured systemd unit or a direct uvicorn invocation
would bypass that — now the lifespan itself refuses, before any
worker, DB or bus comes up.
Resolve DECNET_JWT_SECRET eagerly at startup so a missing or known-
bad value fails at boot rather than on the first auth-gated request.
The lazy-load shape stays useful for non-master CLIs.
