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.

Judgement call: messages also give the detail's file, source, error and mode.
Judgement call: alerts_suppressed_total is the same for every destination.
Judgement call: an alerts.json with waiting as a list stops the start.

Model: opus-5-5

internal/lookup/lookup_test.go

@@ -243,7 +243,7 @@ func TestFailureRaisesASourceFailureAlertOncePerCooldown(t *testing.T) {
 		wantCountry(t, g, clients(), "")
 		wantRequests(t, geojs, 2)
 
-		waiting := queue.Snapshot().Waiting
+		waiting := queue.Snapshot().Waiting[alerts.DestinationWebhook]
 		if len(waiting) != 1 || !reflect.DeepEqual(waiting[0], want) {
 			t.Errorf("alerts waiting %+v, want only %+v", waiting, want)
 		}