feat(m6): board with threads/comments, markdown, tests, e2e
- BoardRepository (thread/comment CRUD, soft-delete, search, pagination,
pinned-first ordering) + BoardService (membership permission,
author/admin edit/delete, comment->board_commented notification,
board_posted/comment_added activity logs, category validation)
- APIs: threads list/create/detail/edit/delete, comments create/edit/delete
- MarkdownBody (react-markdown + remark-gfm + rehype-sanitize),
ThreadForm, CommentForm, board list + thread detail screens, ProjectNav link
- Unit tests (BoardRepository, BoardService) + e2e board.spec
2017585f84