feat: Add cameras[].media.severities to filter review media by severity

Review queries were issued without a severity filter, so the thumbnail
bar, gallery and timeline showed every Frigate review item -- alerts and
detections alike -- regardless of what a camera is configured to alert
on. Severity could only be narrowed interactively, in the media filter
drawer, and that choice did not persist.

`media.severities` resolves into the default query the same way
`media.reviewed` does, and is overridden by an explicit severity filter
so the media filter drawer keeps working. Unset means all severities, so
existing behaviour is unchanged.

    cameras:
      - camera_entity: camera.office
        media:
          severities:
            - high

tests/view/utils/query-filter.test.ts

@@ -6,6 +6,7 @@ import type { UnifiedQuery } from '../../../src/view/unified-query';
 import {
   getReviewedQueryFilterFromConfig,
   getReviewedQueryFilterFromQuery,
+  getSeverityQueryFilterFromConfig,
 } from '../../../src/view/utils/query-filter';
 import { createFolder } from '../../test-utils';
 import { createEventQuery, TestViewMedia } from '../test-utils';
@@ -81,4 +82,21 @@ describe('query-filter', () => {
       expect(getReviewedQueryFilterFromConfig('unreviewed')).toBe(false);
     });
   });
+
+  describe('getSeverityQueryFilterFromConfig', () => {
+    it('should return undefined when unset', () => {
+      expect(getSeverityQueryFilterFromConfig(undefined)).toBeUndefined();
+    });
+
+    it('should return undefined when empty', () => {
+      expect(getSeverityQueryFilterFromConfig([])).toBeUndefined();
+    });
+
+    it('should return a set of the configured severities', () => {
+      expect(getSeverityQueryFilterFromConfig(['high'])).toEqual(new Set(['high']));
+      expect(getSeverityQueryFilterFromConfig(['high', 'medium'])).toEqual(
+        new Set(['high', 'medium']),
+      );
+    });
+  });
 });