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

lib/api/services.ts

@@ -7,6 +7,8 @@ import { UserRepository } from '@/repositories/UserRepository';
 import { ProjectService } from '@/services/ProjectService';
 import { NotificationService } from '@/services/NotificationService';
 import { ActivityLogService } from '@/services/ActivityLogService';
+import { BoardService } from '@/services/BoardService';
+import { BoardRepository } from '@/repositories/BoardRepository';
 
 /**
  * Route Handler用に各Repository/Serviceを構築するファクトリ。
@@ -30,6 +32,16 @@ export function createActivityLogService(): ActivityLogService {
   return new ActivityLogService(new ActivityLogRepository(getDb()));
 }
 
+export function createBoardService(): BoardService {
+  const db = getDb();
+  return new BoardService(
+    new BoardRepository(db),
+    new ProjectMemberRepository(db),
+    new NotificationService(new NotificationRepository(db)),
+    new ActivityLogService(new ActivityLogRepository(db))
+  );
+}
+
 export function createUserRepository(): UserRepository {
   return new UserRepository(getDb());
 }