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

src/config/schema/cameras.ts

@@ -223,6 +223,10 @@ const cameraMediaConfigSchema = z.object({
   reviewed: z
     .enum(CAMERA_MEDIA_REVIEWED_FILTERS)
     .default(cameraMediaConfigDefault.reviewed),
+
+  // Restrict review media to these severities (e.g. `[high]` for Frigate
+  // alerts only). Unset means all severities.
+  severities: severitySchema.array().min(1).optional(),
 });
 
 export const cameraConfigSchema = z