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

app/projects/[projectId]/board/[threadId]/page.tsx

@@ -0,0 +1,78 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser, toPublicUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService } from '@/lib/api/services';
+import { Header } from '@/components/layout/Header';
+import { ProjectNav } from '@/components/layout/ProjectNav';
+import { MarkdownBody } from '@/components/board/MarkdownBody';
+import { CommentForm } from '@/components/board/CommentForm';
+import { ForbiddenError, NotFoundError } from '@/lib/errors';
+
+export const dynamic = 'force-dynamic';
+
+export default async function ThreadDetailPage({
+  params,
+}: {
+  params: Promise<{ projectId: string; threadId: string }>;
+}) {
+  const user = await getCurrentUser();
+  if (!user) redirect('/login');
+  const { projectId, threadId } = await params;
+
+  const boardService = createBoardService();
+  let thread;
+  let comments;
+  try {
+    thread = boardService.getThread(user.id, Number(threadId));
+    comments = boardService.listComments(user.id, Number(threadId));
+  } catch (error) {
+    if (error instanceof ForbiddenError || error instanceof NotFoundError) {
+      redirect(`/projects/${projectId}/board`);
+    }
+    throw error;
+  }
+
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <Header user={toPublicUser(user)} />
+      <ProjectNav projectId={Number(projectId)} active="board" />
+      <main className="mx-auto max-w-3xl space-y-6 p-6">
+        <a
+          href={`/projects/${projectId}/board`}
+          className="text-sm text-blue-600 hover:underline"
+        >
+          ← 掲示板一覧へ
+        </a>
+        <div className="rounded-lg border bg-white p-6 shadow-sm">
+          <div className="flex items-center justify-between">
+            <h1 className="text-2xl font-bold">{thread.title}</h1>
+            {thread.category && (
+              <span className="rounded bg-gray-100 px-2 py-0.5 text-xs">
+                {thread.category}
+              </span>
+            )}
+          </div>
+          <div className="mt-4">
+            <MarkdownBody bodyMd={thread.bodyMd} />
+          </div>
+          <p className="mt-4 text-xs text-gray-400">
+            投稿: {thread.createdAt} / 更新: {thread.updatedAt}
+          </p>
+        </div>
+
+        <section className="space-y-4">
+          <h2 className="text-lg font-bold">コメント ({comments.total})</h2>
+          {comments.items.map((comment) => (
+            <div
+              key={comment.id}
+              className="rounded-lg border bg-white p-4 shadow-sm"
+            >
+              <MarkdownBody bodyMd={comment.bodyMd} />
+              <p className="mt-2 text-xs text-gray-400">{comment.createdAt}</p>
+            </div>
+          ))}
+          <CommentForm projectId={Number(projectId)} threadId={thread.id} />
+        </section>
+      </main>
+    </div>
+  );
+}