Backend: Add database schema for storing incidents (or notice) data

[aldy505]

On Railway status page (https://status.railway.app/) they call it "notice" instead of "incident", since not everything is an incident anyway. Maybe we should convert our wording into that.

But.. we still need a database schema for storing those notices. It should include this data (as minimum):

• Title -> the title of the notice
• Timestamp -> Event timestamp
• Status -> must be one of "investigating" | "identified" | "monitoring" | "resolved" | "scheduled". If the timestamp is in the future, the status must only be "scheduled"
• Level (or severity) -> Informational, Warning, Error, Fatal

Some examples on how we would display this on the frontend:

Incident detail: