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/rules/rules_test.go

@@ -387,7 +387,7 @@ func TestBrokenEditKeepsTheRulesAsTheyWere(t *testing.T) {
 	wantFileError := func() {
 		t.Helper()
 
-		waiting := queue.Snapshot().Waiting
+		waiting := queue.Snapshot().Waiting[alerts.DestinationWebhook]
 		if len(waiting) != 1 || waiting[0].Event != alerts.EventFileError ||
 			waiting[0].Reason != hasError || waiting[0].Detail["error"] != want ||
 			waiting[0].Detail["file"] != filepath.Join(dir, firstFile) {