security(relay): minor hardening follow-ups from PR review

- Uniform auth responses remove the session-enumeration oracle: unknown
  session and wrong password now return an identical 401 at the API layer and
  an identical WS handshake/close; desktop-status and the 503 "not connected"
  signal are only exposed after successful auth.
- Session-count cap re-checked after bcrypt.hash so concurrent creates can't
  overshoot maxSessions.
- Relay web client reconnect backoff resets only after a successful auth
  response (an accept-then-close server no longer defeats the backoff).
- idGenerator comment corrected to match the rejection-sampling; drop unused
  recordFailure return value.
- DEPLOY.md: document ALLOWED_ORIGINS for reverse-proxy deployments.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

macropad-relay/src/handlers/webClientHandler.ts

@@ -32,31 +32,19 @@ export function handleWebClientConnection(
   // client IP. This prevents one abusive IP (which knows the shareable session
   // id) from locking out other web clients or the desktop for this session.
   const throttleKey = `web:${sessionId}:${clientIp}`;
-  // Check if session exists
-  const session = sessionManager.getSession(sessionId);
-  if (!session) {
-    socket.send(JSON.stringify({
-      type: 'error',
-      error: 'Session not found'
-    }));
-    socket.close();
-    return;
-  }
 
-  // Add client (not authenticated yet)
+  // Do NOT reveal whether the session exists. A non-existent session id and
+  // an existing one must be indistinguishable to an unauthenticated client so
+  // session ids cannot be enumerated: we always run the same handshake and let
+  // the (uniform) auth response be the only signal. validatePassword() safely
+  // returns false for an unknown session id, and desktop connectivity is only
+  // disclosed AFTER a successful auth (see the 'auth' case below).
   const client = connectionManager.addWebClient(sessionId, socket, false);
 
   // Track failed auth attempts on this individual socket.
   let socketAuthFailures = 0;
 
-  // Check if desktop is connected
-  const desktop = connectionManager.getDesktopBySessionId(sessionId);
-  socket.send(JSON.stringify({
-    type: 'desktop_status',
-    status: desktop ? 'connected' : 'disconnected'
-  }));
-
-  // Request authentication
+  // Request authentication.
   socket.send(JSON.stringify({
     type: 'auth_required'
   }));
@@ -76,6 +64,12 @@ export function handleWebClientConnection(
             }
           } else {
             socketAuthFailures = 0;
+            // Authenticated: only now do we disclose desktop connectivity.
+            const desktop = connectionManager.getDesktopBySessionId(sessionId);
+            socket.send(JSON.stringify({
+              type: 'desktop_status',
+              status: desktop ? 'connected' : 'disconnected'
+            }));
           }
           break;
         }
@@ -142,7 +136,7 @@ async function handleAuth(
     socket.send(JSON.stringify({
       type: 'auth_response',
       success: false,
-      error: 'Invalid password'
+      error: 'Authentication failed'
     }));
     return true;
   }