retry the remaining plain schedules with backon

hive-forge-notify/Cargo.toml

@@ -15,6 +15,7 @@ workspace = true
 
 [dependencies]
 anyhow.workspace = true
+backon.workspace = true
 forgejo-api.workspace = true
 hive-agent-sock.workspace = true
 hive-sock-client.workspace = true
@@ -26,3 +27,8 @@ tokio.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 url.workspace = true
+
+[dev-dependencies]
+# `test-util` for `#[tokio::test(start_paused = true)]`: the token wait is
+# thirty seconds a read, ten minutes in all.
+tokio = { workspace = true, features = ["test-util"] }