hub: serve the Hub on its own port instead of through Caddy

Caddy fronted the Hub at http://sovransystemsos.local, but the Hub
already listens on 0.0.0.0:8937 itself, and nothing Caddy added is
something it needs:

- Not the name. That is avahi's: mDNS advertises a hostname, not a port,
  so the name resolves wherever the Hub listens.
- Not TLS (the site was plain http), not authentication, not cache
  headers. The header block duplicated NoCacheMiddleware, and its
  Clear-Site-Data ("cache") overrode the app's stronger ("cache",
  "storage").
- Not access control, and this is the point. With ports 80/443 forwarded
  for public services, a Host header on those ports reached the Hub. That
  second door is how the reported bug happened, and c33457f guards it
  with an address check instead of closing it.

The Hub is now served on port 8937 only, at
http://sovransystemsos.local:8937, and Caddy has no site for it. The
only thing Caddy answers on 80/443 is the public sites. Caddy keeps
Ride The Lightning (:3051) and Mempool (:60847), because those do need
it: Sovran_Bitcoin binds both to 127.0.0.1 and RTL's unit is sandboxed to
loopback besides, so Caddy is how the local network reaches them.

- caddy.nix: no Hub site. Caddy runs wherever RTL and Mempool do, which
  includes Bitcoin Node Only. There it did not run at all (enable was
  needsHttpsPorts || extraVhosts != ""), so :3051 and :60847 were open
  in the firewall with nothing listening. Ports 80/443 still follow
  needsHttpsPorts alone, so Node Only does not open them. The two sites
  are written only where their service exists; they were unconditional.
- sovran-hub.nix: 8937 follows the new hub.directPort, 60847 follows
  Mempool. It used to be `[ 8937 60847 ]` on every role, Desktop Only
  included.
- roles.nix: hub.directPort defaults to !roles.desktop: open on Server +
  Desktop and Bitcoin Node Only, closed on Desktop Only, where the Hub is
  reached from the machine itself through the desktop window on
  localhost.
- The bind stays 0.0.0.0, which is IPv4 only: with that bind [::1]:8937
  is refused and "localhost" falls back to 127.0.0.1. That is on purpose
  and is now said in the comment. An IPv6 listener would let in clients
  whose global addresses the Hub cannot tell from a stranger's, which is
  the question the previous commit declines to answer by guessing.
- README, SECURITY.md and two strings in index.html give the new URL.

Behaviour changes: the Hub's address gains :8937, and http://sovransystemsos.local
on port 80 no longer reaches it. Bitcoin Node Only now runs Caddy.

Evaluated with nix eval (nixpkgs as flake.lock pins it, Sovran_Bitcoin at
the locked revision), firewall TCP ports per role:

                      c33457f                      this commit
  Server + Desktop    22 80 443 3051 8937 60847    22 80 443 3051 8937
  Bitcoin Node Only   22 3051 8937 60847           22 3051 8937 60847   (Caddy now runs)
  Desktop Only        22 8937 60847                22

Port 22 is open on every role although sshd listens on loopback only;
the last commit of this series deals with that.

The Caddyfile the module really generates (the evaluated generator
script, run, then `caddy validate` with Caddy 2.9.1): Node Only gets the
two sites and nothing else; Server + Desktop with every domain
configured gets the seven domain sites plus :3051 and :60847 and no
mention of the Hub; with Bitcoin off there are no local-network sites;
Node Only with Bitcoin off and no domains leaves Caddy off.

Add tests/test_hub_direct.py and keep tests/test_caddy_lan_only.py for
the two sites it still covers.

modules/core/roles.nix

@@ -84,6 +84,27 @@
         '';
       };
 
+      directPort = lib.mkOption {
+        type = lib.types.bool;
+        default = !config.sovran_systemsOS.roles.desktop;
+        defaultText = lib.literalExpression "!config.sovran_systemsOS.roles.desktop";
+        description = ''
+          Open port 8937 on the firewall, so that other devices on the local
+          network can reach the Hub at http://sovransystemsos.local:8937.
+
+          On by default for Server + Desktop and Bitcoin Node Only. Off on
+          Desktop Only, the role most likely to be used away from home: there
+          nothing is published, and the Hub is reachable only from this
+          computer, through the desktop application window on localhost. Set it
+          to true in custom.nix if you do want to reach a Desktop Only Hub from
+          another device.
+
+          The Hub runs as root, so it checks every client itself (see
+          sovran_systemsOS.hub.lanOnly); the firewall opening only decides
+          whether a packet may reach it at all.
+        '';
+      };
+
       extraLanNetworks = lib.mkOption {
         type = lib.types.listOf lib.types.str;
         default = [ ];