phase 6: container events + drop the 5s /api/state poll

new DashboardEvent::ContainerStateChanged + ContainerRemoved
close the last refetch loop on the dashboard. Coordinator's
rescan_containers_and_emit diffs a fresh container_view::build_all
against a cached last_containers map and fires per-row events.
called from actions::approve (post-spawn), actions::destroy,
the lifecycle_action wrapper, auto_update::rebuild_agent, and
the existing 10s crash_watch poll.

ContainerView extracted to its own module so coordinator and
dashboard can both build it. dashboard endpoints flip to 200;
container-lifecycle forms carry data-no-refresh. client drops
the periodic poll entirely — initial cold load + SSE for
everything afterwards. pending overlay reads from the existing
transientsState since the new event payload doesn't carry it.

PURG3 + meta-update keep the post-submit refetch since
tombstones + meta_inputs aren't event-derived yet; tracked in
TODO.md.

hive-c0re/src/actions.rs

@@ -85,6 +85,10 @@ pub async fn approve(coord: Arc<Coordinator>, id: i64) -> Result<()> {
                 if let Err(e) = finish_approval(&coord_bg, &approval_bg, result, None) {
                     tracing::warn!(agent = %agent_bg, error = ?e, "spawn approval failed");
                 }
+                // New container row appeared (or didn't, on failure
+                // before nixos-container create completed) — rescan so
+                // dashboards reflect the post-spawn state.
+                coord_bg.rescan_containers_and_emit().await;
             });
             Ok(())
         }
@@ -355,6 +359,9 @@ pub async fn destroy(coord: &Arc<Coordinator>, name: &str, purge: bool) -> Resul
     coord.notify_manager(&HelperEvent::Destroyed {
         agent: name.to_owned(),
     });
+    // Container row disappeared — rescan so the dashboard fires
+    // `ContainerRemoved` for the gone row.
+    coord.rescan_containers_and_emit().await;
     Ok(())
 }