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

ROADMAP.md

@@ -107,6 +107,12 @@ external interfaces. See `docs/architecture/structure.canvas` → *Backend – M
       a dead consumer never acknowledged; nodes that reach outside are skipped on a
       redelivery they already ran. Long-lived worker pools replace the per-wave
       executors, and a delay now waits in the queue rather than on a worker thread
+- [x] Engine history in Postgres: 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 — including the manual runs and previews that never went through the
+      queue. Read back through `/observability/*`, which always answers 200 so a
+      degraded engine still renders, and pruned on a retention window
 - [ ] Test nodes: a small node dragged onto an existing one, smoke or unit, blocking
       deployment on failure
 - [ ] User management scoped per flow and per data set
@@ -169,6 +175,10 @@ React + Vite, primarily desktop but usable on mobile. See `docs/architecture/str
       channels/rules each get a sidebar page, and the OAuth clients an agent
       registers are listed and revocable under Admin — which needed its
       management endpoints written first
+- [x] Health screen: how the engine is doing now (nodes, flows, queue, loop lag)
+      over what it has been doing all day — throughput and failure charts, a
+      per-flow table, the recent cascades, failures that expand to their
+      traceback, dead-lettered work and the audit trail
 - [x] Mobile-friendly canvas: touch connect, full-screen node panel
 - [ ] Installable as a PWA (`vite-plugin-pwa`)