notify: a reply other than 2xx is a failed delivery (closes #287)

A notification endpoint that answered with a redirect was counted as
delivered: the request is not sent through a client, so the redirect
is never followed, and only a status of 400 or more counted as a
failure. Now any status outside 200-299 fails the delivery for ntfy,
Slack and Mattermost, so it is retried and logged like the others.

Model: opus-5-5

internal/notify/retry_test.go

@@ -536,3 +536,33 @@ func TestSendNotificationPermanentFailureLogsError(
 		return attempts >= 3
 	})
 }
+
+// TestSendNotificationRedirectFails checks that a 302 reply is a
+// failed delivery at each endpoint: the server is sent the first
+// attempt and the retry, and each endpoint logs a retry line and
+// then the final failure line, both naming the 302.
+func TestSendNotificationRedirectFails(t *testing.T) {
+	t.Parallel()
+
+	var requests atomic.Int64
+
+	srv := httptest.NewServer(
+		http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				requests.Add(1)
+				http.Redirect(w, r, "/moved", http.StatusFound)
+			}),
+	)
+	defer srv.Close()
+
+	logs := sendToEachEndpoint(t, srv.URL, "t")
+
+	// ntfy, Slack and Mattermost each send once and retry once.
+	const wantRequests = 3 * 2
+
+	if sent := requests.Load(); sent != wantRequests {
+		t.Errorf("server got %d requests, want %d", sent, wantRequests)
+	}
+
+	assertEachEndpointRetriedThenFailed(t, logs, "status 302")
+}