fix(sub): send stable X-HWID on external subscription fetch

A Master panel fetching a donor subscription sent no X-HWID, so an
HWID-limited donor rejected it with 404. Identify this panel with a
stable per-installation id (persisted in settings), occupying exactly
one donor device slot.

Fixes MHSanaei/3x-ui#6559

internal/sub/external_hwid_test.go

@@ -0,0 +1,43 @@
+package sub
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/database"
+	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
+)
+
+// #6559: the Master panel must send a stable X-HWID when fetching external
+// subscriptions, otherwise an HWID-limited donor answers 404.
+func TestServerHwidStableAcrossCalls(t *testing.T) {
+	if err := database.InitDB(filepath.Join(t.TempDir(), "x-ui.db")); err != nil {
+		t.Fatalf("InitDB: %v", err)
+	}
+	t.Cleanup(func() { _ = database.CloseDB() })
+
+	first := serverHwid()
+	if first == "" {
+		t.Fatal("serverHwid returned empty")
+	}
+	if !strings.HasPrefix(first, "3x-ui-server-") {
+		t.Fatalf("unexpected hwid format: %q", first)
+	}
+	if len(first) < 6 {
+		t.Fatalf("hwid too short for donor minHwidLength: %q", first)
+	}
+
+	second := serverHwid()
+	if second != first {
+		t.Fatalf("hwid not stable: %q vs %q", first, second)
+	}
+
+	var row model.Setting
+	if err := database.GetDB().Where("key = ?", serverHwidKey).First(&row).Error; err != nil {
+		t.Fatalf("hwid not persisted: %v", err)
+	}
+	if row.Value != first {
+		t.Fatalf("persisted hwid %q != returned %q", row.Value, first)
+	}
+}