Lone-worker safety check-ins

BC's Workers' Compensation rules require periodic check-ins for guards working alone. To enable:

• On the site's edit page, set "Lone-worker interval" (in minutes, e.g. 60 or 90).
• That's it. The /api/cron/lone-worker job runs every 5 min. Whenever an IN_PROGRESS shift at that site has no partner on duty AND the configured interval has elapsed since the last prompt, a new prompt is generated.
• The guard sees a "Confirm you're OK" banner on the PWA home; one tap acks it.
• If the prompt is unacked 5 minutes past its scheduled time, the cron fires a CUSTOM alert (payload.topic="lone_worker") tagged for dispatch escalation.

Existing escalation routing (alert emails, SMS where Twilio is configured) carries the lone-worker alerts the same way panic alerts flow.