fix(udp): read WSJT-X packets that arrive through a relay

A forwarder (W&P, seen in the field in front of MSHV) prepends the origin as
plain text before re-broadcasting:

    "127.0.0.1:2237|" + <the original, untouched WSJT-X packet>

That puts the magic 15 bytes in, so every datagram failed on "bad magic
0x3132372e" — those four bytes being ASCII "127." — and an operator running
MSHV behind the relay saw no decodes, no callsigns and no auto-logged QSOs.

No new service type: what follows the header IS a WSJT-X packet, so the parser
and everything downstream apply unchanged, and a separate type would duplicate
decode, status and logged-ADIF handling to strip 15 bytes. ParseWSJT skips the
header instead, which also covers any other relay that wraps traffic this way.

The match is deliberately narrow — the magic must fall within the first 64
bytes AND every byte before it must be printable ASCII. A corrupt or truncated
packet that merely contains those four bytes somewhere is not resurrected into
a QSO; it fails exactly as it did before.

Test data is the real captured datagram, header included.

internal/integrations/udp/wsjt.go

@@ -56,12 +56,61 @@ type WSJTEvent struct {
 	IsCQ        bool   // the decode was a CQ call
 }
 
+// maxFwdHeader bounds how far into a packet the WSJT-X magic may sit behind a
+// forwarder's header. The one seen in the field ("127.0.0.1:2237|") is 15 bytes;
+// 64 leaves room for a longer address without ever scanning a real payload.
+const maxFwdHeader = 64
+
+// stripForwarderHeader removes the origin header a UDP relay prepends.
+//
+// A relay that re-broadcasts WSJT-X traffic has to say where each datagram came
+// from, and it does so as plain text in front of the payload:
+//
+//	"127.0.0.1:2237|" + <the original, untouched WSJT-X packet>
+//
+// The magic then sits 15 bytes in, every packet fails on "bad magic", and an
+// operator running MSHV behind such a relay gets nothing at all. There is no
+// need for a separate service type: what follows the header IS a WSJT-X packet,
+// so the whole parser and everything downstream apply unchanged.
+//
+// Deliberately narrow. The magic must appear within maxFwdHeader bytes AND
+// everything before it must be printable ASCII — a truncated or corrupt packet
+// that happens to contain those four bytes somewhere is not resurrected into a
+// QSO. Anything else is returned untouched, and still fails as it did.
+func stripForwarderHeader(pkt []byte) []byte {
+	if len(pkt) < 4 {
+		return pkt
+	}
+	if binary.BigEndian.Uint32(pkt) == wsjtMagic {
+		return pkt // no header — the overwhelmingly common case
+	}
+	limit := len(pkt) - 4
+	if limit > maxFwdHeader {
+		limit = maxFwdHeader
+	}
+	for i := 1; i <= limit; i++ {
+		if binary.BigEndian.Uint32(pkt[i:]) != wsjtMagic {
+			continue
+		}
+		for _, b := range pkt[:i] {
+			if b < 0x20 || b >= 0x7f {
+				return pkt // not a text header — leave it alone
+			}
+		}
+		return pkt[i:]
+	}
+	return pkt
+}
+
 // ParseWSJT decodes one UDP packet. Returns ok=false for messages we
 // don't care about (heartbeat, clears, etc.).
 func ParseWSJT(pkt []byte) (WSJTEvent, bool, error) {
 	if len(pkt) < 12 {
 		return WSJTEvent{}, false, fmt.Errorf("packet too short")
 	}
+	// A relay (W&P and friends) puts its own origin header in front — skip it so
+	// the packet parses exactly as if it had arrived from WSJT-X directly.
+	pkt = stripForwarderHeader(pkt)
 	r := bytes.NewReader(pkt)
 	var magic, schema, mtype uint32
 	if err := binary.Read(r, binary.BigEndian, &magic); err != nil {