[social] Follows, likes, posts, feed, discover

- Database: `follows`, `likes`, `posts` tables

- Post types: site published (auto), page added (auto), image shared, text post, repost

- Following feed (content from people you follow)

- Discover feed (trending/new content across platform)

- Like button on sites, pages, images

- Follow/unfollow on user profiles

- Basic in-app notification system

**Files:** Supabase migration, new components in `src/components/social/`