refactor(#2591): DagView carries host-computed timestamps + NodeView.has_log

Reconcile with mara + argus's review on the frontend PR (#2660):

- DagView regains started_at/finished_at (DateTime<Utc>), computed
  host-side as min/max over ALL subtree nodes (including the Done ones
  filtered off the wire). The client can't derive these — the
  earliest/only-started node is often Done and absent — so the backend
  sets them, per mara's call.
- NodeView gains has_log: bool = build_log_id.is_some(), the precise
  old 'node has a captured build log' guard so the dashboard only shows
  a log link for nodes that actually produce one.

hive-sh4re/src/jobs.rs

@@ -121,6 +121,12 @@ pub struct NodeView {
     /// node. Display-only payload, not derivable from the graph.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub inputs: Vec<String>,
+    /// Whether this node has a captured build log fetchable at
+    /// `GET /api/build-log/<id>`. Only the nix-heavy nodes that stream build
+    /// output set one; the client gates its log link on this so lock / noop /
+    /// store-only nodes don't render a link that 404s.
+    #[serde(default)]
+    pub has_log: bool,
 }
 
 /// A queued / running / failed DAG — a thin projection of one container
@@ -137,6 +143,17 @@ pub struct DagView {
     pub reason: String,
     /// When the DAG was enqueued.
     pub created_at: DateTime<Utc>,
+    /// When the DAG's first node started (min over *all* its nodes) — computed
+    /// host-side, **not** derived on the client: `Done` nodes are excluded from
+    /// `nodes` below, so the earliest-started node is usually absent from the
+    /// wire and the client can't take the min itself. `None` until a node runs.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub started_at: Option<DateTime<Utc>>,
+    /// When the DAG finished (max `finished_at` over all its nodes), set only
+    /// once the DAG has settled terminal. Host-computed for the same reason as
+    /// `started_at`. `None` while the DAG is still live.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub finished_at: Option<DateTime<Utc>>,
     /// Nodes of this DAG with `Done` ones excluded. A DAG whose nodes are
     /// all `Done` is omitted from the snapshot entirely; a `Failed` DAG
     /// lingers until aged out by the history cap.