feat(notifications): add a failed-check threshold for node.down alerts (#6768)

One lost heartbeat on a lossy link flipped a node offline and sent node.down,
and the next probe sent node.up, so a flaky node flooded Telegram, email and
Discord with alert pairs.

A new nodeDownThreshold setting (default 1, today's behaviour) works like
outboundDownThreshold: node.down fires once the failed-check streak reaches it,
once per outage, and node.up only after a node.down was sent. The announcement
is persisted in nodes.down_notified, so a restart inside the threshold window
neither drops the outage's node.down nor re-announces nodes already reported
offline; nodes offline at upgrade are seeded as announced.

internal/database/node_down_notified_migration_test.go

@@ -0,0 +1,43 @@
+package database
+
+import (
+	"testing"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/config"
+	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
+)
+
+func TestNodeDownNotifiedSeederMarksOfflineNodesAnnounced(t *testing.T) {
+	t.Setenv("XUI_DB_FOLDER", t.TempDir())
+	if err := InitDB(config.GetDBPath()); err != nil {
+		t.Fatalf("init db: %v", err)
+	}
+	t.Cleanup(func() { _ = CloseDB() })
+	if err := db.Where("seeder_name = ?", "NodeDownNotified").Delete(&model.HistoryOfSeeders{}).Error; err != nil {
+		t.Fatalf("clear seeder history: %v", err)
+	}
+	offline := model.Node{Name: "offline", Address: "10.0.0.1", Port: 1, Status: "offline"}
+	online := model.Node{Name: "online", Address: "10.0.0.2", Port: 1, Status: "online"}
+	for _, n := range []*model.Node{&offline, &online} {
+		if err := db.Create(n).Error; err != nil {
+			t.Fatalf("create node %s: %v", n.Name, err)
+		}
+	}
+
+	if err := runSeeders(false); err != nil {
+		t.Fatalf("runSeeders: %v", err)
+	}
+
+	for _, tc := range []struct {
+		node *model.Node
+		want bool
+	}{{&offline, true}, {&online, false}} {
+		var got model.Node
+		if err := db.First(&got, tc.node.Id).Error; err != nil {
+			t.Fatalf("reload node %s: %v", tc.node.Name, err)
+		}
+		if got.DownNotified != tc.want {
+			t.Errorf("node %s DownNotified = %v, want %v", tc.node.Name, got.DownNotified, tc.want)
+		}
+	}
+}