Add WebSocket maxPayload limit (64KB) (FRE-4747)

Set maxPayload: 65536 on WebSocketServer constructor to bound
per-message memory usage, addressing security review
recommendation M1 from FRE-4474.

services/spamshield/src/websocket/alert-server.ts

@@ -58,6 +58,7 @@ export class AlertServer {
     this.wss = new WebSocketServer({
       port: this.config.port,
       host: this.config.host,
+      maxPayload: 65536, // 64KB limit to prevent memory exhaustion attacks
     });
     
     this.setupWebSocketHandlers();