Keep the engine's own history, and a screen that reads it

A second bus subscriber folds executions, errors, timings and queue lag
into per-minute rollups, keeps failures with their traceback and an audit
trail of who published what, and records one row per cascade — manual runs
and previews included, under an id of their own that writes no idempotency
markers. Read back through /observability/*, which always answers 200 so a
degraded engine still renders its own health screen.

Also fixes two things found on the way: node-health alerts read `status`
where the engine publishes `health`, so a device dropping never alerted
anyone, and the Redis queue reported `parked: 0` whatever was held.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

frontend/src/components/Sidebar/AppSidebar.tsx

@@ -1,4 +1,5 @@
 import {
+  Activity,
   Bell,
   Home,
   KeyRound,
@@ -26,6 +27,7 @@ const baseItems: Item[] = [
   { icon: Home, title: "Home", path: "/" },
   { icon: Workflow, title: "Flows", path: "/flows" },
   { icon: LayoutDashboard, title: "Dashboards", path: "/dashboards" },
+  { icon: Activity, title: "Health", path: "/health" },
   // Both are engine-wide operator settings rather than personal ones, so they
   // sit here and not among the per-user tabs under Settings.
   { icon: KeyRound, title: "Secrets", path: "/secrets" },