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/

frontend/index.html

@@ -46,6 +46,10 @@
     </div>
     <p id="live-data-status-today-paused" class="live-data-status-today-paused" role="status" hidden>Today paused while you check live data.</p>
     <div id="live-data-status-feeds" class="live-data-status-feeds"></div>
+    <div id="bucket-latency-panel" class="bucket-latency-panel" aria-labelledby="bucket-latency-heading" hidden>
+      <h3 id="bucket-latency-heading" class="small muted">Bucket aggregation latency</h3>
+      <div id="bucket-latency-rows" role="list"></div>
+    </div>
     <p id="live-data-status-retry" class="small muted"></p>
     <div class="live-data-status-actions">
       <button id="refresh-live-data" type="button">Refresh now</button>