jobq: stamp created_at on every node, beside started_at/finished_at

`Node` carried two of the three lifecycle timestamps; the third lived on
hive-c0re's `NodeKind::Dag` container payload, a core-specific wrapper the
graph knows nothing about. Give it its real home so the container's copy
becomes redundant rather than load-bearing.

Not an `Option` like its neighbours: starting and finishing are events that
may never happen, but a node that exists was created. Modelling it as
optional would encode a state the graph cannot be in.

`hive-jobq-wire::GraphNode` gains the field in the same commit — it already
carries the other two, and without this one the value cannot reach a viewer
when the container's copy is deleted.

hive-jobq-wire/src/lib.rs

@@ -152,6 +152,9 @@ pub struct GraphNode {
     /// What must hold before this node runs. Omitted when empty.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub deps: Vec<GraphDep>,
+    /// When the node was inserted into the graph. Always present — a node that
+    /// exists was created, so unlike the two below this is not an `Option`.
+    pub created_at: DateTime<Utc>,
     /// When the node entered `Running`. `None` until it starts; a node that
     /// never ran keeps `None`.
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -254,6 +257,7 @@ fn wire_node<N: WireNode, R: WireResource>(node: &hive_jobq::Node<N, R>) -> Grap
         parent: node.parent.map(NodeId::get),
         state: node.state,
         deps: node.deps.iter().map(wire_dep).collect(),
+        created_at: node.created_at,
         started_at: node.started_at,
         finished_at: node.finished_at,
         error: node.error.clone(),
@@ -443,6 +447,7 @@ mod tests {
             parent,
             state,
             deps,
+            created_at: chrono::DateTime::<chrono::Utc>::default(),
             started_at: None,
             finished_at: None,
             error: None,