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.
02e2bf895e