feat: bucket aggregation latency panel (Closes #11)

Measure each live aggregation bucket's own upstream duration server-side
(auth fetch attributed to Work/Activity, per-feed timed gather) and surface
the measured values in the Live data status panel.

- Per-bucket latency_ms measured individually, never batch wall time
- Stale feeds label their number 'last known'; unmeasured feeds say
  'not measured' instead of an ambiguous dash; hidden when no data
- Failed buckets keep their last known latency across partial refreshes
- Panel is aria-labelled rows as a list, values wrap on mobile
- Ignore .venv/

.gitignore

@@ -1,5 +1,6 @@
 __pycache__/
 *.py[cod]
 .pytest_cache/
+.venv/
 .release-engine/
 .stackchain-state/