feat(outbounds): support custom subscription user agents (#6398)

Some subscription providers require a client-specific User-Agent before returning outbound links. Persist an optional value per subscription and use it for refreshes and previews while preserving the existing default for blank values.

internal/database/db.go

@@ -96,10 +96,21 @@ func migrateClientTrafficLastSubFetchColumn() error {
 	return migrator.AddColumn(&xray.ClientTraffic{}, "LastSubFetch")
 }
 
+func migrateOutboundSubscriptionUserAgentColumn() error {
+	migrator := db.Migrator()
+	if !migrator.HasTable(&model.OutboundSubscription{}) || migrator.HasColumn(&model.OutboundSubscription{}, "user_agent") {
+		return nil
+	}
+	return migrator.AddColumn(&model.OutboundSubscription{}, "UserAgent")
+}
+
 func initModels() error {
 	if err := migrateClientTrafficLastSubFetchColumn(); err != nil {
 		return err
 	}
+	if err := migrateOutboundSubscriptionUserAgentColumn(); err != nil {
+		return err
+	}
 	models := allModels()
 	for _, mdl := range models {
 		if IsPostgres() && postgresModelSettled(mdl) {