Goal: Let users filter the event log by one or more severities via the toolbar.Acceptance criteria:

• The Event Log toolbar has a Severity filter control next to existing filters (Event, “Filter by event”, time range).

• Severity options come from the backend: 

  GET /notifications/severities

  Only show options returned by the API

• User can select "Severity" then "filter by severity" and select one or more options. Once selected you will see similar to the attached image.
• If user selects a filter and no choices exist, show loading/error state (img attached of an example)
• Clearing the severity filter removes the severities param and shows all events again. Filter state is consistent with URL/query params if the app uses them for the Event Log.
• Add storybook test