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/amneziawg/manager_test.go

@@ -156,6 +156,21 @@ func TestStructuralFingerprintStableAndSensitive(t *testing.T) {
 	if a.structuralFingerprint() != c.structuralFingerprint() {
 		t.Fatal("a peer-only change must NOT change the structural fingerprint")
 	}
+
+	d := baseInstance()
+	d.IPv6Enabled = true
+	if a.structuralFingerprint() == d.structuralFingerprint() {
+		t.Fatal("enabling IPv6 must change the structural fingerprint")
+	}
+
+	e := baseInstance()
+	e.IPv6Enabled = true
+	f := baseInstance()
+	f.IPv6Enabled = true
+	f.IPv6ExternalInterface = "eth1"
+	if e.structuralFingerprint() == f.structuralFingerprint() {
+		t.Fatal("changing IPv6ExternalInterface must change the structural fingerprint -- otherwise the edit is a complete no-op")
+	}
 }
 
 func TestPeersFingerprintOrderIndependentButContentSensitive(t *testing.T) {
@@ -231,6 +246,12 @@ func TestHostRulesFingerprintCoversForwardedPortsAndPeerIP(t *testing.T) {
 	if a.hostRulesFingerprint() == fewer.hostRulesFingerprint() {
 		t.Fatal("removing a peer must change the host-rules fingerprint -- one fewer TPROXY rule is needed")
 	}
+
+	ip6Added := baseInstance()
+	ip6Added.Peers[0].AllowedIPs = []string{"10.8.1.2/32", "fd86:ea04:1115::2/128"}
+	if a.hostRulesFingerprint() == ip6Added.hostRulesFingerprint() {
+		t.Fatal("adding a peer's IPv6 address must change the host-rules fingerprint -- its NDP-proxy entry is keyed on it, and a change here must force the full bounce that (re-)runs PostUp")
+	}
 }
 
 func TestRouteEgressComment(t *testing.T) {
@@ -308,6 +329,9 @@ func TestDefaultPostUpDownEmitsTproxyForEveryPeer(t *testing.T) {
 	if !strings.Contains(up, fmt.Sprintf("ip rule add fwmark %#x", EgressFwmark)) {
 		t.Errorf("expected the shared policy route to be added once in PostUp, got:\n%s", up)
 	}
+	if wantCheck := fmt.Sprintf("ip rule list | grep -q 'fwmark %#x lookup %d'", EgressFwmark, EgressTable); !strings.Contains(up, wantCheck) {
+		t.Errorf("expected an existence check before 'ip rule add', so repeated bounces don't accumulate duplicate rules, got:\n%s", up)
+	}
 	if strings.Contains(down, "ip rule") || strings.Contains(down, "ip route") {
 		t.Error("the shared policy route must never be removed in PostDown -- other instances may still need it")
 	}