Make queue_worker health check reflect worker liveness, not scheduler

[morcen]

What was broken

/health/detailed's queue_worker status is meant to indicate whether webhook deliveries are actively being processed, but it only proved the scheduler container's cron was alive — not that the queue container (which runs queue:work and executes SendWebhook) was running at all.

The queue:heartbeat artisan command wrote the queue:heartbeat Redis key directly, and was itself only ever invoked by the scheduler's cron (routes/console.php). The scheduler and queue containers are separate processes, so if the queue container crashed, OOMed, or its queue:work process died while scheduler kept running, /health/detailed would keep reporting "queue_worker": "ok" — a false positive in exactly the failure mode it exists to catch. Every triggered webhook would silently queue up and never be delivered, with no monitoring signal at all.

What changed

• Added App\Jobs\WriteQueueHeartbeat, a queued job dispatched onto the same webhooks queue SendWebhook runs on. Its handle() is what now writes the queue:heartbeat Redis key.
• QueueHeartbeat (the artisan command the scheduler runs every minute) now dispatches WriteQueueHeartbeat::dispatch() instead of writing the key itself.

Since the heartbeat key is only written once an actual queue worker pulls WriteQueueHeartbeat off the webhooks queue and executes it, staleness now reflects whether a worker is alive and processing jobs — not just whether the scheduler container is ticking. A dead queue:work process now correctly surfaces as queue_worker: stale within the existing 2-minute staleness window, even while the scheduler keeps running.

• Updated CLAUDE.md, ARCHITECTURE.md, DEPLOYMENT.md, and WEBHOOK_README.md, which previously documented the heartbeat as a scheduler-liveness signal.
• Added tests/Feature/QueueHeartbeatTest.php covering: the command dispatches the job onto the webhooks queue, the command itself never writes the Redis key directly, the job writes the current timestamp when handled, and the job is queued on webhooks.

Testing

• vendor/bin/pint --dirty — clean
• php artisan test — full suite passes (256 passed, 1 pre-existing skip)

Fixes #108