Render unknown for a zero CreatedAt in Slack/Mattermost messages (closes #298)

FormatSlackMessage rendered the Timestamp line unconditionally, so a
reaped-row fallback delivery carried "0001-01-01T00:00:00Z" -- the
exact string #257 was filed
against, indistinguishable from that regression. When
event.CreatedAt.IsZero(), render "unknown" instead; a real CreatedAt
still renders as RFC3339.

internal/delivery/target_slack.go

@@ -231,10 +231,15 @@ func FormatSlackMessage(
 		event.ContentType,
 	)
 
+	timestamp := "unknown"
+	if !event.CreatedAt.IsZero() {
+		timestamp = event.CreatedAt.UTC().Format(time.RFC3339)
+	}
+
 	fmt.Fprintf(
 		&b,
 		"*Timestamp:* `%s`\n",
-		event.CreatedAt.UTC().Format(time.RFC3339),
+		timestamp,
 	)
 
 	fmt.Fprintf(