Critical fixes for feature enforcement:
- Add GET /api/features/{feature_id}/check-access endpoint (was missing!)
- Add migration for missing features: data_export, csv_import
- These features were used in frontend but didn't exist in DB
This fixes:
- "No analysis available" when setting KI limit
- Export features not working
- Frontend calling non-existent API endpoint
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
