feat: public-facing status page for external stakeholders

[metapragma]

Add a public /status page that displays system health and incident history, something like status.github.com or status.stripe.com.

Who would this be useful for?

• Customers who want to check if there are known issues before contacting support
• Tenant admins who want to share a status URL with my users
• Operators who want incidents to automatically reflect on the public page

Possible features.

• Public route no auth required
• Current status banner (All Systems Operational/Partial Outage/etc.)
• 90-day uptime bars visualization
• Active incidents with real-time updates
• Incident history timeline

Also consider subdomain support (status.{tenant}.signalgrid.io).