feat: add posts, tags database schema and API
- Add migration 002_posts.sql with posts, tags, post_tags tables
- Add Post/Tag/PostStats models with PostStatus enum
- Add posts API with full CRUD:
- create_post, update_post, delete_post (admin only)
- get_post_by_slug, list_published_posts (public)
- list_posts, get_post_stats (admin)
- list_tags, get_posts_by_tag, search_posts (public)
- Slug auto-generation with uniqueness check
- Server-side markdown rendering with pulldown-cmark
- Auto-summary extraction from markdown
- Soft delete support
xfy
973d6f3d57