Telemetry: boot-slow beacon names the 3 slowest resources

The first boot-slow capture (5763ms total, html 68ms) proved the white screen
happens AFTER the shell arrives — but not which fetch eats the time. Append
the 3 slowest resource entries (path, start→end, transferSize; sz0 ≈ served
from SW/cache) so the next slow boot names its culprit. Reason cap 300→500
client+server to fit the detail.

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

goodnews/api.py

@@ -948,7 +948,7 @@ def create_app() -> FastAPI:
         with get_conn() as conn:
             conn.execute(
                 "INSERT INTO client_errors (reason, path, user_agent, app_version) VALUES (?, ?, ?, ?)",
-                ((body.reason or "")[:300], (body.path or "")[:200], ua, (body.version or "")[:60]),
+                ((body.reason or "")[:500], (body.path or "")[:200], ua, (body.version or "")[:60]),
             )
             conn.execute("DELETE FROM client_errors WHERE created_at < datetime('now','-14 days')")
             conn.commit()