[grade=B] fix(auth): reuse valid session for cross-host SSO

dashcaddy-api/routes/auth/sso-gate.js

@@ -203,8 +203,22 @@ module.exports = function(deps) {
     }
   }, 'auth-app-token'));
 
+  // A browser that already has a valid status.sami session must not be asked
+  // for TOTP again just because it opened another private-TLD service host.
+  // Mint a fresh one-time token that the target host can exchange for its own
+  // host-only cookie. This route is intentionally session-protected both by
+  // the global middleware and here (defence in depth).
+  router.get('/auth/sso-handoff', (req, res) => {
+    res.setHeader('Cache-Control', 'no-store');
+    if (!session.isValid(req)) {
+      return errorResponse(res, 401, 'Session expired or invalid');
+    }
+    ok(res, { ssoToken: session.createHandoffToken() });
+  });
+
   // Cross-subdomain SSO handoff: exchanges a short-lived single-use token
-  // (minted by /totp/verify) for a HOST-ONLY session cookie on whichever
+  // (minted by /totp/verify or /auth/sso-handoff) for a HOST-ONLY session
+  // cookie on whichever
   // *.sami origin calls this. Needed because Domain=.sami cookies are
   // silently rejected by real browsers (.sami is an unregistered TLD, so
   // browsers treat "sami" as the effective public suffix and refuse to set