bfcache: server - Implement WebSocket server functionality

* Added WebSocketServer class with configurable options
* Introduced WebSocketServerFactory for creating WebSocket relay servers
* Updated index.ts to set up a server with WebSocket support and handle various signals for graceful shutdown

Web/bfcache/server/src/net/websockets/WebSocketServerFactory.ts

@@ -0,0 +1,37 @@
+import {LoggerFactory} from '@techniker-me/logger';
+import WebSocketServer, {WebSocketServerOptions} from './WebSocketServer';
+import {ServerWebSocket} from 'bun';
+
+export default class WebSocketServerFactory {
+  public static createWebSocketRelayServer(): WebSocketServer {
+    const logger = LoggerFactory.getLogger('WebSocketRelayServer');
+    const clients = new Set<ServerWebSocket>();
+    const webSocketRelayServerOptions: WebSocketServerOptions = {
+      onSocketError: (client, error) => logger.error(`Error: [%o] [${error.message}]`, client),
+      onSocketOpen: client => {
+        logger.debug('New WebSocketClient [%o]', client);
+
+        clients.add(client);
+      },
+      onSocketMessage: (fromClient, message) => {
+        logger.debug(`Relaying message [%o]`, message);
+
+        for (const client of clients) {
+          if (client === fromClient) {
+            continue;
+          }
+
+          client.send(message);
+        }
+      },
+      onSocketClose: client => clients.delete(client),
+      onSocketDrain: client => logger.debug('Client drain [%o]', client),
+      publishToSelf: false
+    };
+
+    return new WebSocketServer(webSocketRelayServerOptions);
+  }
+  private constructor() {
+    throw new Error('WebSocketServerFactory is a static class that may not be instantiated');
+  }
+}