hub: answer the local network only, whichever way a client arrives

The reported bug was the Hub being reachable from outside the local
network when Server + Desktop is active. c33457f guards the Caddy site
for sovransystemsos.local, but the Hub is an application that also
listens on a port of its own (8937), and a check in Caddy does nothing
for a client that never goes through Caddy. Whether a client could reach
the Hub depended on which door it used.

The Hub now asks the question itself. LanOnlyMiddleware is registered
outermost, so a client that is not on this computer or the local network
gets a bare 403 before authentication is considered; it never sees the
login page.

- Local means loopback, 10/8, 172.16/12, 192.168/16, 100.64/10
  (Tailscale and other CGNAT/VPN ranges) and 169.254/16 over IPv4, and
  ::1, fc00::/7 and fe80::/10 over IPv6.
- IPv6 global addresses are not on the list. A global address belonging
  to a laptop on the LAN cannot be told apart from a stranger's by the
  address alone, and 2000::/3 is every public IPv6 address there is.
- ::ffff:a.b.c.d is read as the IPv4 address inside it.
- sovran_systemsOS.hub.extraLanNetworks adds networks (IPv4 or IPv6 CIDR)
  for setups whose own devices use addresses outside those ranges. It is
  checked at build time. The app ignores an entry it cannot parse and
  refuses 0.0.0.0/0 and ::/0: it must never widen its policy by
  guessing, and "everyone" is hub.lanOnly = false, asked for by name.
- sovran_systemsOS.hub.lanOnly (default true) turns the check off.
- The first refusal from each address is logged, naming the option to
  change, so an operator whose own device is refused can find out why.
  The list is capped so a scanner cannot fill the journal or memory.

Behind Caddy the policy applies to the real client, not to Caddy: uvicorn
takes the address from X-Forwarded-For only when the peer is 127.0.0.1.

Checked, not only reviewed:
- The real app with the config.json the module really generates (nix
  eval on the nixpkgs revision flake.lock pins, read back from the
  derivation), on a real socket with the source address chosen per
  request: 127.0.0.1, 192.168/16, 100.64/10 and a declared extra
  network are served; 203.0.113.9, 8.8.4.4 and an address just outside
  the declared /28 get 403 on /login, / and /api/ping. /auto-login still
  answers 303 to loopback and 403 to everyone else.
- With c33457f's Caddy guard in front, an IPv6 client at 2001:db8::9
  passes Caddy (it is inside 2000::/3) and is refused by the Hub; an
  IPv4 stranger has the connection closed by Caddy; a LAN client is
  served.
- hub.extraLanNetworks accepts 203.0.113.0/28, 2001:db8:abcd::/48 and
  bare hosts, and fails the build for /33, 300.1.1.1/8, 0.0.0.0/0, ::/0,
  2001:db8::/129 and junk, with a message that says what to write.

Add tests/test_lan_policy.py and tests/test_hub_lan_only.py, and a note
in SECURITY.md.

app/sovran_systemsos_web/server.py

@@ -56,6 +56,7 @@ from .security_helpers import (
     load_session_store,
     save_session_store,
     LoginThrottle,
+    LanPolicy,
     LOGIN_FAIL_DELAY,
     LOGIN_FAIL_MAX_DELAY,
     LOGIN_FAIL_WINDOW,
@@ -821,8 +822,61 @@ class AuthMiddleware(BaseHTTPMiddleware):
         return await call_next(request)
 
 
+# ── Local-network middleware ───────────────────────────────────
+#
+# The Hub runs as root. Whether a packet may reach its port is up to the
+# firewall and the router; this is the second lock, so a port forward or a
+# firewall mistake does not put the login page in front of the internet. It
+# runs before authentication: a client that is not on the local network never
+# sees the login page at all.
+#
+# Built from the Nix-generated config. lan_only defaults to True, so a Hub built
+# without the key still turns off-network clients away rather than failing open.
+_hub_cfg = load_config()
+_lan_policy = LanPolicy(
+    enabled=bool(_hub_cfg.get("lan_only", True)),
+    extra_networks=tuple(_hub_cfg.get("lan_extra_networks") or ()),
+)
+
+
+class LanOnlyMiddleware(BaseHTTPMiddleware):
+    """Refuse clients that are not on this computer or the local network."""
+
+    # Each refused address is logged once, and the list is capped: a scanner
+    # must not be able to fill the journal or the process's memory.
+    _MAX_LOGGED = 256
+
+    def __init__(self, app, policy):
+        super().__init__(app)
+        self._policy = policy
+        self._logged: set = set()
+
+    async def dispatch(self, request: Request, call_next):
+        client_ip = request.client.host if request.client else None
+        if not self._policy.allows(client_ip):
+            self._note_refusal(client_ip)
+            return JSONResponse(
+                {"detail": "Not available from this network"}, status_code=403,
+            )
+        return await call_next(request)
+
+    def _note_refusal(self, client_ip):
+        if client_ip in self._logged or len(self._logged) >= self._MAX_LOGGED:
+            return
+        self._logged.add(client_ip)
+        logger.warning(
+            "Refused a Hub request from %r: not this computer or a local "
+            "network. If that address is yours, add its network to "
+            "sovran_systemsOS.hub.extraLanNetworks.",
+            client_ip,
+        )
+
+
 app.add_middleware(AuthMiddleware)
 app.add_middleware(NoCacheMiddleware)
+# Registered last so it runs outermost: a client that is not on the local
+# network is turned away before authentication is considered at all.
+app.add_middleware(LanOnlyMiddleware, policy=_lan_policy)
 
 _ICONS_DIR = os.environ.get(
     "SOVRAN_HUB_ICONS",