Add gateway realtime fanout and cursor pagination docs

lib/server.dart

@@ -3,6 +3,7 @@ import 'package:relic/relic.dart';
 
 import 'config/config.dart';
 import 'src/auth/auth_middleware.dart';
+import 'src/gateway/gateway_handlers.dart';
 import 'src/handlers/auth_handlers.dart';
 import 'src/handlers/channel_handlers.dart';
 import 'src/handlers/message_handlers.dart';
@@ -26,6 +27,7 @@ Future<void> startServer() async {
     ..get('/.well-known/users/:username', publicUserHandler)
     ..get('/.well-known/verify/:userId', publicVerifyHandler)
     ..get('/.well-known/server', publicServerInfoHandler)
+    ..get('/gateway', gatewayHandler)
 
     // ============ PROTECTED ENDPOINTS (requires auth) ============
     ..use('/api', jwtMiddleware)