A guest login: temporary, look-only, revocable

Users -> Guest logins makes a throwaway user name and password, good for half
an hour by default, to hand to somebody who wants to see the console. It is a
normal account with `guest: true` and an expiry, so login, sessions and the
saved look need no special case, and it is left out of the account list, out of
"the last account can't be removed", and out of the one account an empty user
name means.

What a guest may do is one allow-list, checked in the auth middleware, rather
than a flag ~190 handlers have to remember. The list is matched by asking the
router which handler a request would reach, so it cannot drift from the routes
it names, and it is checked against the scoped path — /api/nodes/grid/procs is
/api/procs on grid — because the hub proxies to a node with a token that is
full control of that node. The node cannot tell a guest from its owner, so the
only place it can be stopped is here, before the request leaves.

A guest gets the dashboard, containers, stacks, disks, hardware, ZFS and usage,
on this machine and on any managed one, and nothing else: no terminal (attaching
is a GET, and it is a root shell), no files, no container logs or environments,
no stack files, no alert channels, no cron, no tokens, no settings, no power.

Expiry is enforced on every request and swept once a minute, so a console left
open on a visitor's phone stops within the minute rather than at their next
click; revoking signs them out at once. The console hides what a guest cannot
use and says how long is left where the power button would be.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -54,6 +54,15 @@ reset a password; your own password confirms the last two). From the shell:
 users` lists them, `hamlab user-rm <user>` removes one. Machine settings — refresh rate,
 graph history, alert rules — stay the machine's and are shared by everyone.
 
+**Guest logins**, for showing somebody the console: Users → Guest logins makes a
+throwaway user name and password, good for half an hour by default, and hands you the
+pair to paste into a message. A guest sees the dashboard, the containers, the disks, the
+hardware and the machines this hub manages, and can change none of it — no terminal, no
+files, no logs or container environments, no settings, no power. It can be revoked in one
+tap, and it disappears on its own when the time is up, wherever it is signed in. What a
+guest may reach is a list of routes in `internal/api/guests.go`, so a route added later
+is shut to guests until somebody says otherwise.
+
 What adapts to the host, rather than assuming Debian:
 
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