feat: add WebSocket gateway on port 18789

JSON protocol server using esp_http_server with WS upgrade.
Supports up to 4 concurrent clients, auto-assigned chat_id,
routes messages through the agent via message bus.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main/gateway/ws_server.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Initialize and start the WebSocket server on MIMI_WS_PORT.
+ * Allows external clients to interact with the Agent via JSON messages.
+ *
+ * Protocol:
+ *   Inbound:  {"type":"message","content":"hello","chat_id":"ws_client1"}
+ *   Outbound: {"type":"response","content":"Hi!","chat_id":"ws_client1"}
+ */
+esp_err_t ws_server_start(void);
+
+/**
+ * Send a text message to a specific WebSocket client by chat_id.
+ * @param chat_id  Client identifier (assigned on connection)
+ * @param text     Message text
+ */
+esp_err_t ws_server_send(const char *chat_id, const char *text);
+
+/**
+ * Stop the WebSocket server.
+ */
+esp_err_t ws_server_stop(void);