Boot-failure seatbelt: no future crash becomes a silent white screen

Per Codex. A branded recovery card in app.html shows if the app hasn't mounted
in 7s, or on a pre-mount JS error/unhandledrejection — with a "Refresh Upbeat
Bytes" button. A chunk/preload failure (vite:preloadError) reloads once
(sessionStorage-guarded). +layout calls window.__ubBooted() on mount to clear
the card + timer. A pre-mount failure also fires a tiny anonymous client_error
beacon; the admin Overview now shows "Load errors today" (red if >0) so we can
see if blank-risk is happening in the wild.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/api.py

@@ -444,6 +444,7 @@ _EVENT_KINDS = {
     "share_ub", "copy_source", "native_share",
     "not_today", "less_like_this", "hide_topic",
     "replace_used", "replace_none", "paywall_replace", "paywalled_source_open",
+    "client_error",  # boot-failure seatbelt beacon (blank-screen risk signal)
 }