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_subscription.go

@@ -9,6 +9,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/google/uuid"
 	"github.com/mhsanaei/3x-ui/v3/internal/database"
 	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
 	"github.com/mhsanaei/3x-ui/v3/internal/logger"
@@ -150,6 +151,12 @@ func doFetchSubscriptionLinks(rawURL string) ([]string, error) {
 	}
 	// Some providers gate the link body on a known client User-Agent.
 	req.Header.Set("User-Agent", "v2rayNG/1.8.5")
+	// A 3x-ui donor with an HWID limit answers 404 when the header is empty
+	// (#6559). Identify this panel with a stable per-installation id so the
+	// donor registers exactly one device slot for it.
+	if hwid := serverHwid(); hwid != "" {
+		req.Header.Set("X-HWID", hwid)
+	}
 	resp, err := subscriptionHTTPClient.Do(req)
 	if err != nil {
 		return nil, err
@@ -173,6 +180,36 @@ var (
 	errSubscriptionBodyTooLarge = &subError{"subscription response body exceeds size limit"}
 )
 
+// serverHwidKey is the settings row holding this panel's stable identity for
+// outbound external-subscription fetches.
+const serverHwidKey = "externalSubHwid"
+
+// serverHwid returns a stable per-installation id, creating and persisting it
+// on first use. A random-per-request value would burn one donor HWID slot per
+// fetch; empty means the DB is unreachable, in which case no header is sent.
+func serverHwid() string {
+	db := database.GetDB()
+	if db == nil {
+		return ""
+	}
+	var row model.Setting
+	if err := db.Where("key = ?", serverHwidKey).First(&row).Error; err == nil {
+		if strings.TrimSpace(row.Value) != "" {
+			return strings.TrimSpace(row.Value)
+		}
+	}
+	hwid := "3x-ui-server-" + uuid.NewString()
+	row = model.Setting{Key: serverHwidKey, Value: hwid}
+	if err := db.Where(model.Setting{Key: serverHwidKey}).FirstOrCreate(&row).Error; err != nil {
+		logger.Warningf("sub: persisting server hwid failed: %v", err)
+		return ""
+	}
+	if strings.TrimSpace(row.Value) == "" {
+		return hwid
+	}
+	return strings.TrimSpace(row.Value)
+}
+
 type subError struct{ msg string }
 
 func (e *subError) Error() string { return e.msg }