Implement a new feature that generates thumbnail previews for image uploads. The change adds a dedicated preview handler that processes uploaded images, creates scaled-down versions, and stores them alongside the original files. This enables faster loading of image galleries and list views without fetching full-resolution assets.
- Added BlockingService class with methods for managing blocked users and content
- Integrated blocking checks into message delivery pipeline to filter blocked senders
- Added database migration for blocked_users table with user_id and target_id columns
