Alerts to Slack and ntfy, each destination with its own queue (closes #90)

Each alert is posted as a message to the Slack incoming webhook
SWWAF_ALERT_SLACK_WEBHOOK_URL names, and published to the ntfy topic
SWWAF_ALERT_NTFY_URL names, with SWWAF_ALERT_NTFY_TOKEN as a bearer
token and a priority and tag by event. The cooldown and the hourly
limit stay shared; past them, each destination has its own bounded
queue and backoff, and its own sent, failed and dropped counts.
alerts.json keeps the alerts waiting by destination; one whose waiting
is still a list stops the start, saying what to change. A control
character in the ntfy token, or in the instance name ntfy is sent,
stops the start.

Judgement call: messages also give the detail's file, source, error and mode.
Judgement call: alerts_suppressed_total is the same for every destination.

Model: opus-5-5

internal/proxy/alerts_test.go

@@ -124,7 +124,7 @@ func TestObserveModeRaisesTheBanAlertsItWouldHave(t *testing.T) {
 			"for the attack alone, as it was", held, line.BanExpires)
 	}
 
-	waiting := queue.Snapshot().Waiting
+	waiting := queue.Snapshot().Waiting[alerts.DestinationWebhook]
 	if len(waiting) != 3 || queue.Suppressed() != 0 {
 		t.Fatalf("%d alerts wait and %d are held back, want 3 and 0: %+v",
 			len(waiting), queue.Suppressed(), waiting)
@@ -189,7 +189,8 @@ func TestObserveModeWorksOutABanOnlyWhenItsAlertWouldBeSent(t *testing.T) {
 	// Had the ban been worked out for any of the requests within the
 	// cooldown or past the two an hour, its alert would have been raised,
 	// held back and counted.
-	if waiting := queue.Snapshot().Waiting; len(waiting) != 2 || queue.Suppressed() != 0 {
+	waiting := queue.Snapshot().Waiting[alerts.DestinationWebhook]
+	if len(waiting) != 2 || queue.Suppressed() != 0 {
 		t.Errorf("%d alerts wait and %d are held back, want 2 and 0: %+v",
 			len(waiting), queue.Suppressed(), waiting)
 	}
@@ -250,7 +251,7 @@ func attackAlert(
 func wantAlerts(t *testing.T, queue *alerts.Queue, want ...alerts.Alert) {
 	t.Helper()
 
-	got := queue.Snapshot().Waiting
+	got := queue.Snapshot().Waiting[alerts.DestinationWebhook]
 	if len(got) != len(want) {
 		t.Fatalf("%d alerts wait, want %d: %+v", len(got), len(want), got)
 	}