Reliability: slow ≠ failed — SW nav timeout, slow-boot telemetry, de-bot stats

Root cause of the intermittent white screen: the shell HTML is no-cache
(cf-cache-status: DYNAMIC), so every page-open does a synchronous round-trip
to the residential origin before any pixel renders — and the SW's network-first
navigation only fell back to the cached shell on REJECTION, never on slowness.
A stalled fetch meant staring at white with a perfectly good shell in cache.
The boot seatbelt couldn't see it either: it lives inside the HTML that hadn't
arrived yet, so slow boots left no telemetry.

- service-worker: race navigation fetch vs 2.5s grace timer. Network wins →
  fresh HTML as before; timer/5xx/failure → cached shell instantly, network
  response still refreshes the cache in the background. Safe due to the 14-day
  immutable-chunk grace window. Caps the white screen at ~2.5s for repeat
  visitors on any network.
- app.html: beacon `boot-slow: Nms (html Nms) on 4g` when mount takes >4s —
  the "white screen, then it loaded" glitches finally leave a trace, with
  HTML-arrival timing to separate slow-origin from slow-JS.
- admin: bot UAs (HeadlessChrome/bot/spider/crawl/…) excluded from the
  headline "Load errors today" count — throttled crawlers trip the 10s boot
  check routinely (the one recorded error was HeadlessChrome on X11, not a
  phone). Bots stay visible in the list, tagged + dimmed.

Tests: telemetry test extended for bot flag + filtered counts. 223 pytest +
11 vitest green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app.html

@@ -61,6 +61,21 @@
           var el = document.getElementById('boot-fallback');
           if (el && el.parentNode) el.parentNode.removeChild(el);
           try { sessionStorage.removeItem('ub_reloaded'); } catch (e) {}
+          // Slow-but-successful boots (the "white screen, then it loaded" glitch)
+          // would otherwise leave no trace — beacon the timing so they're visible.
+          // performance.now() counts from navigation start, so a slow-arriving
+          // HTML document is included, not just slow JS.
+          try {
+            var ms = Math.round(performance.now());
+            if (ms > 4000) {
+              var nav = performance.getEntriesByType && performance.getEntriesByType('navigation')[0];
+              var detail = 'boot-slow: ' + ms + 'ms';
+              if (nav && nav.responseStart) detail += ' (html ' + Math.round(nav.responseStart) + 'ms)';
+              if (navigator.connection && navigator.connection.effectiveType)
+                detail += ' on ' + navigator.connection.effectiveType;
+              report(detail);
+            }
+          } catch (e) { /* timing is best-effort */ }
         };
         addEventListener('vite:preloadError', function (e) {
           report('preloadError: ' + ((e && e.payload && e.payload.message) || ''));