- Force sanic.root/error/server back to INFO from before_server_start, after
dev mode's runtime setLevel(DEBUG) — kills the useless 'Error Page:' noise
- Silence mediapreview.pool WARNINGs (timeouts already in access log extra);
they previously fell to logging.lastResort with no level prefix
- Handle CancelledError with 499 (client disconnect / RequestCancelled) or
503 (server shutdown) instead of Sanic's default 500 error page
- 422 preview failures now log 'backend: reason' in the access log, with
upstream error text shortened (first line, no [Errno] prefix, cut at ': ');
dev mode prints the full original error to console