feat: tags on posts

[srmdn]

Summary

Allow posts to be tagged. Tags are filterable in the API and can be used by themes to group posts.

Scope

• Add tags column to posts table (comma-separated or join table — TBD at implementation)
• Tag input in the post editor (admin UI)
• GET /api/posts supports ?tag= filter param
• GET /api/posts/:slug includes tags in response
• GET /api/tags returns all tags with post counts (public, for theme nav/sidebars)

Out of scope

• Tag pages in theme (theme concern)
• Tag search / autocomplete in editor (can add later)