improving-drf-endpoints

Use when editing, reviewing, or auditing DRF viewsets and serializers in PostHog. Triggers on files in posthog/api/, products/*/backend/api/, products/*/backend/presentation/, or any file importing rest_framework serializers or viewsets. Covers OpenAPI spec quality, field typing, schema annotations, and DRF best practices that flow through the type pipeline to generated TypeScript types and MCP tools.