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/schemas/protocols/inbound/mtproto.ts

@@ -22,5 +22,12 @@ export const MtprotoInboundSettingsSchema = z.object({
   preferIp: z.enum(['prefer-ipv6', 'prefer-ipv4', 'only-ipv6', 'only-ipv4']).optional(),
   debug: z.boolean().optional(),
   domainFronting: MtprotoDomainFrontingSchema.optional(),
+  // When set, the mtg sidecar dials Telegram through a loopback SOCKS bridge in
+  // the Xray config so the egress obeys routing rules. `outboundTag` optionally
+  // forces that traffic out a specific outbound/balancer. `routeXrayPort` is the
+  // bridge port; it is allocated and owned by the backend (never edited here).
+  routeThroughXray: z.boolean().optional(),
+  outboundTag: z.string().optional(),
+  routeXrayPort: z.number().int().min(0).max(65535).optional(),
 });
 export type MtprotoInboundSettings = z.infer<typeof MtprotoInboundSettingsSchema>;