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.

frontend/src/api/queries/useOutboundTags.ts

@@ -0,0 +1,33 @@
+import { useQuery } from '@tanstack/react-query';
+
+import { keys } from '@/api/queryKeys';
+import { fetchXrayConfig } from '@/hooks/useXraySetting';
+
+// Available outbound (and balancer-eligible) tags the user can route an mtproto
+// inbound's Telegram traffic to. Shares the cached xray config query so opening
+// the inbound form costs no extra request when the Xray page was already
+// visited; `select` derives just the tag list without disturbing other readers.
+export function useOutboundTags() {
+  return useQuery({
+    queryKey: keys.xray.config(),
+    queryFn: fetchXrayConfig,
+    staleTime: Infinity,
+    select: (data): string[] => {
+      const tags = new Set<string>();
+      for (const o of data?.xraySetting?.outbounds ?? []) {
+        const tag = (o as { tag?: string } | null)?.tag;
+        if (tag) tags.add(tag);
+      }
+      for (const t of data?.subscriptionOutboundTags ?? []) {
+        if (t) tags.add(t);
+      }
+      // Balancers are valid routing targets too — injectMtprotoEgress emits a
+      // balancerTag rule when the chosen tag names a balancer.
+      const balancers = (data?.xraySetting?.routing as { balancers?: Array<{ tag?: string }> } | undefined)?.balancers;
+      for (const b of balancers ?? []) {
+        if (b?.tag) tags.add(b.tag);
+      }
+      return [...tags];
+    },
+  });
+}