fix(amneziawg): resolve 7 Medium findings from the automated PR review

Each is independently reproducible; fixed together since one review pass
found all of them.

- manager.go: the shared "ip rule add fwmark" policy route had no
  existence check, so it duplicated in "ip rule show" on every interface
  bounce (which hostRulesFingerprint forces on any client add/remove/
  re-IP). Now checked via "ip rule list | grep -q ..." first. (Finding 2)

- params.go: ExternalInterface, IPv6ExternalInterface, and subnetIp/
  subnetCidr are interpolated unescaped into a shell-executed PostUp/
  PostDown line, but only obfuscation and the IPv6 subnet were validated
  before save. Added ValidateInterfaceName (a strict charset+length
  pattern) and ValidateSubnetIPv4 (netip.ParsePrefix), wired into
  normalizeAmneziaWGSettings. (Finding 3)

- amneziawg_job.go: IsAwgInstalled() existed but nothing ever called it,
  so a host without awg/awg-quick (the Docker image, RHEL, Arch, a failed
  install.sh PPA step) logged a reconcile failure every 10s forever. Now
  checked once an inbound actually needs it, warning once instead of
  spamming. (Finding 4)

- client_inbound_apply.go: the WireGuard/AmneziaWG credential
  carry-forward (added so a metadata-only client edit doesn't rotate
  keys) never covered ForwardedPorts, so a partial edit -- an API call or
  Telegram-bot toggle that omits the field -- silently wiped a client's
  port-forwarding spec. Carried forward and written back the same way the
  key fields already are. (Finding 5)

- manager.go: hostRulesFingerprint keyed each peer on its IPv4 address
  only, and structuralFingerprint omitted IPv6Enabled/IPv6ExternalInterface
  entirely, so an IPv6-only change could pick the syncconf reload path
  (which never re-runs PostUp, leaving a stale NDP-proxy entry) or be a
  complete no-op. Both fingerprints now cover the IPv6 fields. (Finding 6)

- port_conflict.go: the AmneziaWG egress bridge (injectAmneziawgEgress)
  binds 127.0.0.1:63100+id with no collision check anywhere, since it
  isn't a database row the ordinary port-conflict query can see -- same
  blind spot the reserved Xray API port already has its own check for.
  Added the equivalent check for the AmneziaWG bridge port. (Finding 7)

- install.sh: install_amneziawg ran unconditionally for every install/
  update, building a DKMS kernel module and enabling host-wide IPv4/IPv6
  forwarding whether or not the feature is ever used. Gated behind a new
  should_install_amneziawg (XUI_INSTALL_AMNEZIAWG=true/false, or an
  interactive y/N prompt defaulting to no). Also replaced the deprecated
  apt-key adv with a dedicated keyring + signed-by= on the Debian branch,
  and guarded its sources.list appends against duplication on a retried
  install. (Finding 8)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/service/client_inbound_apply.go

@@ -643,6 +643,14 @@ func (s *ClientService) UpdateInboundClient(inboundSvc *InboundService, data *mo
 		if clients[0].KeepAlive == 0 {
 			clients[0].KeepAlive = old.KeepAlive
 		}
+		// ForwardedPorts is AmneziaWG-only (WireGuard's own inbound never
+		// reads it), same carry-forward reasoning as the fields above: a
+		// partial edit (e.g. a Telegram-bot enable/expiry toggle, or an API
+		// call that omits the field) must not silently drop a client's
+		// existing port-forwarding spec.
+		if oldInbound.Protocol == model.AmneziaWG && clients[0].ForwardedPorts == "" {
+			clients[0].ForwardedPorts = old.ForwardedPorts
+		}
 	}
 
 	var oldSettings map[string]any
@@ -693,6 +701,9 @@ func (s *ClientService) UpdateInboundClient(inboundSvc *InboundService, data *mo
 				if clients[0].KeepAlive > 0 {
 					newMap["keepAlive"] = clients[0].KeepAlive
 				}
+				if oldInbound.Protocol == model.AmneziaWG && clients[0].ForwardedPorts != "" {
+					newMap["forwardedPorts"] = clients[0].ForwardedPorts
+				}
 			}
 			if oldClientMap != nil && sameClientConfigExceptUpdatedAt(oldClientMap, newMap) {
 				if v, ok2 := oldClientMap["updated_at"]; ok2 {