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/web/service/node.go

@@ -1064,6 +1064,12 @@ func (s *NodeService) ClearNodeDirty(id int, dirtyAt int64) error {
 		Update("config_dirty", false).Error
 }
 
+func (s *NodeService) SetDownNotified(id int, notified bool) error {
+	return database.GetDB().Model(model.Node{}).
+		Where("id = ?", id).
+		Update("down_notified", notified).Error
+}
+
 func (s *NodeService) MarkNodeInboundsAdopted(id int) error {
 	if id <= 0 {
 		return nil