Refactor ShowAudience handling across the application to align with MPAA rating system. Update ShowAudience enum to reflect new ratings (G, PG, PG-13, R, NC-17) and adjust related data models, API types, and frontend components to support nullable audience values. Enhance metadata handling to incorporate content ratings from external sources, ensuring proper audience filtering in scheduling logic. Update documentation to clarify changes in audience categorization and its implications for content management.

frontend/src/features/admin/channels/components/RulesCard.tsx

@@ -15,7 +15,7 @@ import { Label } from '@/shared/ui/label'
 import { updateTemplate } from '../api'
 import { CollapsibleCard } from './CollapsibleCard'
 
-const EMPTY_WINDOW: AudienceWindow = { from: '06:00:00', to: '23:00:00', maxAudience: 'Teen' }
+const EMPTY_WINDOW: AudienceWindow = { from: '06:00:00', to: '23:00:00', maxAudience: 'PG-13' }
 
 /**
  * Правила отбора кандидатов канала (см. 3.8): детское время и потолок повторов. Это жёсткие