Web: prevent redirect server socket exhaustion from stale connections

The HTTP redirect server has max_open_sockets = 2.  A client that
connects and then disappears silently (network drop, browser crash,
mobile radio loss) leaves its socket occupying a slot indefinitely —
the server has no way to detect the loss until it tries to write to
the socket again.

Enable lru_purge_enable on the redirect server so that when both
slots are occupied by such stale connections, the least-recently-used
one is evicted automatically to make room for a new incoming
connection, rather than refusing it outright.

Add a "Connection: close" header to the 302 response as a complementary
measure.  For well-behaved clients this triggers an immediate TCP
teardown after the redirect is received, shrinking the window during
which a connection can turn into a zombie.  Together the two changes
provide defence in depth: "Connection: close" prevents stale connections
from forming in the first place; lru_purge_enable cleans them up when
they do.

src/helpers/web/WebPanelServer.cpp

@@ -2617,6 +2617,7 @@ bool WebPanelServer::start() {
   redirect_config.backlog_conn = 0;
   redirect_config.recv_wait_timeout = 10;
   redirect_config.send_wait_timeout = 10;
+  redirect_config.httpd.lru_purge_enable = true;
   redirect_config.stack_size = kWebServerStackSize;
   redirect_config.task_priority = tskIDLE_PRIORITY + 2;
   redirect_config.core_id = 0;
@@ -2697,6 +2698,7 @@ esp_err_t WebPanelServer::handleHttpRedirect(httpd_req_t* req) {
   httpd_resp_set_status(req, "302 Found");
   httpd_resp_set_hdr(req, "Location", location);
   httpd_resp_set_hdr(req, "Cache-Control", "no-store");
+  httpd_resp_set_hdr(req, "Connection", "close");
   return httpd_resp_send(req, "", 0);
 }