feat(mtproto): route Telegram egress through Xray routing rules

Add a per-inbound "Route through Xray" toggle (off by default) plus an
optional outbound picker on MTProto inbounds. mtg only supports a SOCKS5
upstream, so when enabled the panel injects a loopback SOCKS bridge into
the generated Xray config — tagged with the inbound's own tag — and mtg
dials Telegram through it via a [network] proxies upstream. The router
then governs Telegram egress: matchable in the Routing tab, or forced to a
chosen outbound/balancer via the picker.

- mtproto: Instance carries RouteThroughXray + XrayRoutePort (in the
  fingerprint); InstanceFromInbound parses them; renderConfig emits the
  socks5 [network] upstream; freeLocalPort exported as FreeLocalPort.
- xray.go: injectMtprotoEgress appends the loopback SOCKS bridge and
  prepends an optional inboundTag->outbound/balancer rule, hot-appliable
  like injectPanelEgress.
- inbound.go: backend-owned egress port persisted in settings, allocated
  once and carried across edits (stored value wins); stripped with the
  inert outboundTag when routing is off; allocation failure fails the save;
  routed add/update/del force a config regen.
- mtproto_job: skip folding mtg metrics for routed inbounds (the bridge,
  carrying the inbound tag, is metered by xray_traffic_job) to avoid
  double-counting.
- frontend: toggle + outbound/balancer Select (useOutboundTags) on the
  MTProto form; i18n keys for all locales.

internal/web/translation/en-US.json

@@ -486,6 +486,11 @@
         "mtgProxyProtocolListener": "Accept PROXY protocol (listener)",
         "mtgPreferIp": "IP preference",
         "mtgDebug": "Debug logging",
+        "mtgRouteThroughXray": "Route through Xray",
+        "mtgRouteThroughXrayHint": "Send this proxy's Telegram traffic through Xray so it follows your routing rules. The mtg sidecar dials out via a loopback SOCKS bridge tagged with this inbound's tag; reference that tag in the Routing tab for advanced rules.",
+        "mtgRouteOutbound": "Outbound",
+        "mtgRouteOutboundHint": "Optional. Force Telegram traffic out through this outbound (or balancer). Leave empty to let your routing rules decide.",
+        "mtgRouteOutboundPlaceholder": "Use routing rules",
         "visionTestseed": "Vision testseed",
         "version": "Version",
         "udpIdleTimeout": "UDP idle timeout (s)",