feat(inbounds): add Port-with-Fallback inbound type

Adds a new "portfallback" protocol that emits as a VLESS-TLS inbound
under the hood but is paired with a sidecar table of child inbounds.
Panel auto-builds settings.fallbacks at Xray-config-gen time from the
sidecar — each child's listen+port becomes the fallback dest, with
SNI/ALPN/path/xver match criteria pulled from the row. No more typing
loopback ports by hand or keeping settings.fallbacks in sync.

Backend: new FallbackService (Get/SetChildren, BuildFallbacksJSON);
two new routes (GET/POST /panel/api/inbounds/:id/fallbackChildren);
xray.GetXrayConfig injects fallbacks for PortFallback inbounds; the
inbound model emits protocol="vless" so Xray accepts the config.

Frontend: PORTFALLBACK joins the protocol dropdown; selecting it
shows the standard VLESS controls plus a Fallback Children table
(inbound picker + per-row SNI/ALPN/path/xver). Children are loaded
on edit and replaced atomically on save.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/service/inbound.go

@@ -24,8 +24,9 @@ import (
 )
 
 type InboundService struct {
-	xrayApi       xray.XrayAPI
-	clientService ClientService
+	xrayApi         xray.XrayAPI
+	clientService   ClientService
+	fallbackService FallbackService
 }
 
 func (s *InboundService) runtimeFor(ib *model.Inbound) (runtime.Runtime, error) {