feat(mtproto): expose mtg-multi's per-IP pending-handshake limit

mtg-multi v1.16.0 adds [defense.pending-handshakes] max-per-ip, which
closes new connections from an IP already holding that many
unauthenticated handshakes, so one address cannot pin the workers other
clients need. The MTProto inbound form now carries it as
pendingHandshakesMaxPerIp; the key is written only when set above 0, so
existing inbounds render a byte-identical config and are not restarted.
A change to it moves the structural fingerprint and restarts the sidecar.

dry-run is left out on purpose: it only feeds the pending_handshake_limit
Prometheus metric, which the panel does not surface. The hint warns that
clients behind carrier NAT share the limit.

internal/web/translation/en-US.json

@@ -507,6 +507,8 @@
         "mtprotoFakeTlsDomainHint": "Default FakeTLS domain used to generate a new client's secret. Each client can front its own domain.",
         "mtgThrottleMaxConnections": "Max connections",
         "mtgThrottleMaxConnectionsHint": "Cap concurrent connections across all users with a fair-share limit. 0 disables throttling.",
+        "mtgPendingHandshakesMaxPerIp": "Max pending handshakes per IP",
+        "mtgPendingHandshakesMaxPerIpHint": "Close new connections from an IP that already has this many unfinished handshakes. Clients behind a shared IP (carrier NAT) share the limit. 0 disables it.",
         "mtgAdTagInvalid": "Ad-tag must be exactly 32 hexadecimal characters.",
         "mtgPublicIpv4": "Public IPv4",
         "mtgPublicIpv6": "Public IPv6",