Terminally fail retrying deliveries with a non-retry target type (closes #82) (#104)

A delivery left in `retrying` whose target type was edited to a fire-and-forget
or unknown type was skipped forever by both restart recovery and the retry
sweep. Both paths now record a result row and mark it `failed`.

internal/delivery/export_test.go

@@ -195,6 +195,13 @@ func (e *Engine) ExportRecoverInFlight(
 	e.recoverInFlight(ctx)
 }
 
+// ExportSweepWebhookRetries exposes sweepWebhookRetries.
+func (e *Engine) ExportSweepWebhookRetries(
+	ctx context.Context, webhookID string,
+) {
+	e.sweepWebhookRetries(ctx, webhookID)
+}
+
 // ExportStart exposes start for testing.
 func (e *Engine) ExportStart() {
 	e.start()