Add Post Entity with Database Persistence

[cameronhotchkies]

Create Post domain entity, repository interface, database migration, and jOOQ implementation. Same vertical slice pattern as A1.

✅ Acceptance Criteria

• Integration test: create post → findById returns it with matching fields
• Integration test: update post → findById reflects changes
• Integration test: delete post → findById returns null
• Integration test: listRecent returns posts ordered newest-first
• Creating post with nonexistent authorId fails (FK constraint)

Internal Coding

MP1

• core/posts/Post.kt — entity with id, authorId, title, body, createdAt, updatedAt
• core/posts/PostRepository.kt — interface with findById, create, update, delete, listByAuthor, listRecent
• V002__create_posts_table.sql — migration with author FK, indexes on created_at
• adapters/db/JooqPostRepository.kt — implements PostRepository