feat(summary): revision-gated cache with stale-while-revalidate (STATE-WP-0066)

Replace the fixed 15s TTL on GET /state/summary with per-table revision
watermarks, stale-while-revalidate background refresh, and a progress-tail
section split. SQLAlchemy write hooks invalidate core or progress sections
on mutation. Adds tests, benchmark script, and operator docs.

dashboard/src/docs/live-data.md

@@ -59,6 +59,11 @@ make api      # db + migrate + uvicorn (restarts if already running)
 
 All endpoints are read-only GET requests. The dashboard never writes to the API.
 
+`/state/summary` is revision-cached server-side. Repeated polls with unchanged
+hub data return `X-StateHub-Cache: hit-revision` without rebuilding the full
+snapshot. Prefer `/state/overview` on the Overview page (lighter bounded
+read model).
+
 ---
 
 *Poll interval: 15 s for most pages, 60 s for Overview. Data is refreshed in the background — the page never reloads itself.*