ProdWebContainerSilent and ProdWorkerContainerSilent alert on Docker
container-stdout volume, which isn't a valid liveness signal for either
role: web's real access logging lives in the file-tailed app-json
source, not stdout, and worker's stdout is unconditionally dropped by
Alloy's own relabel rule. Both fired for hours against a healthy prod.
Give web and worker a real audit.log_heartbeat() beat, same pattern the
subscription notifier already uses, tagged daemon=ROLE and unguarded
across every uvicorn worker/replica (a beat is a single local file
append, nothing like the notifier's leader-election cost). Observability
alert rules to consume this land separately.
