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>

goodnews/api.py

@@ -961,7 +961,9 @@ def create_app() -> FastAPI:
             rows = conn.execute(
                 "SELECT reason, path, user_agent, created_at FROM client_errors ORDER BY id DESC LIMIT 20"
             ).fetchall()
-            return [dict(r) for r in rows]
+            # Bots stay visible in the list (tagged) but are excluded from the
+            # headline counts — see queries.admin_stats.
+            return [{**dict(r), "bot": queries.is_bot_ua(r["user_agent"])} for r in rows]
 
     @app.post("/api/feedback")
     def submit_feedback(body: FeedbackBody, request: Request, background_tasks: BackgroundTasks) -> dict: