fix: improve offline check task alerting logic and status updates

[hauju]

Summary

• Decouple device status updates from webhook alerting — offline check task now always marks stale devices as "offline" even when ALERT_WEBHOOK_URL is not configured
• Reduce redundant iteration by collecting stale devices once and reusing for both status update and alert payload construction
• Improve startup logging to clearly indicate whether webhook alerting is active

Test plan

• Verify devices transition to "offline" status without ALERT_WEBHOOK_URL set
• Verify webhook alerts still fire when ALERT_WEBHOOK_URL is configured
• Run cargo test -p ferrite-server

🤖 Generated with Claude Code

[mighty840]

Looks like this works. Can you verify and close the open tasks so we can merge?