feat(xray): add tunnel health monitor (#5480)

* feat(xray): add tunnel health monitor

* fix(tunnelmonitor): reuse netproxy client and init logger in tests

Replace the duplicated newHTTPClient/dialContextWithProxy with netproxy.NewHTTPClient, which centralises the http/https/socks5 handling and avoids the dial-goroutine connection leak on context cancellation. Cap failures at the threshold during cooldown so the counter stays a true consecutive-failure count. Add TestMain to initialise the logger and fix the nil-pointer panic in the success-after-failure path.

* fix(tunnelmonitor): observable recovery, signal headroom, and hardening

Address the remaining review findings on the tunnel health monitor:

- Recovery is now synchronous and observable: the callback calls
  server.RestartXray() directly and returns its error instead of just
  enqueuing SIGUSR1, so a failed restart no longer masks as success and
  arms the cooldown while the tunnel is still down.
- Give the OS signal channel headroom (buffer 8) so producers cannot
  starve a SIGTERM/SIGINT out of the single slot.
- Warn at startup when the monitor is enabled without a proxy, since the
  probe then measures host connectivity rather than the xray tunnel.
- Cap failures at the threshold in the nil-recover branch too, matching
  the cooldown cap.
- Document the XUI_TUNNEL_HEALTH_* vars in .env.example and the README.
- Add tests for status-code classification, Normalize bounds, New proxy
  scheme errors, the recovery-error and nil-recover paths, the cooldown
  cap, and Run context cancellation (coverage 90%).

---------

Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

.env.example

@@ -4,3 +4,16 @@ XUI_LOG_FOLDER=x-ui
 XUI_BIN_FOLDER=x-ui
 XUI_INIT_WEB_BASE_PATH=/
 # XUI_PORT=8080
+
+# Optional tunnel health monitor (disabled by default). It periodically probes a
+# URL and restarts xray-core after repeated failures. Point XUI_TUNNEL_HEALTH_PROXY
+# at a local xray inbound so the probe tests the tunnel; without it the probe only
+# checks host connectivity and a restart will not fix host network issues. A restart
+# drops every connected client.
+# XUI_TUNNEL_HEALTH_MONITOR=true
+# XUI_TUNNEL_HEALTH_PROXY=socks5://127.0.0.1:1080
+# XUI_TUNNEL_HEALTH_URL=https://www.cloudflare.com/cdn-cgi/trace
+# XUI_TUNNEL_HEALTH_INTERVAL=30s
+# XUI_TUNNEL_HEALTH_TIMEOUT=10s
+# XUI_TUNNEL_HEALTH_FAILURES=3
+# XUI_TUNNEL_HEALTH_COOLDOWN=5m