Use type to reflect the actual type of internal event

[nilmerg]

At the moment, internal events use the type name internal which is rather useless for Web.

The type should use specific names to indicate what the event is about.

It's currently used in two cases:

• Incident age thresholds
• Incident reevaluation at startup

There may be more added in the future. We need to make sure that there are no such surprises for Web anymore, @julianbrost suggested to use an enum for event.type to ensure that.