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

components/board/CommentForm.tsx

@@ -0,0 +1,65 @@
+'use client';
+
+import { useState, type FormEvent } from 'react';
+import { useRouter } from 'next/navigation';
+
+export function CommentForm({
+  projectId,
+  threadId,
+}: {
+  projectId: number;
+  threadId: number;
+}) {
+  const router = useRouter();
+  const [bodyMd, setBodyMd] = useState('');
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  async function onSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    setLoading(true);
+    setError(null);
+    const res = await fetch(
+      `/api/projects/${projectId}/board/threads/${threadId}/comments`,
+      {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ bodyMd }),
+      }
+    );
+    setLoading(false);
+    if (res.ok) {
+      setBodyMd('');
+      router.refresh();
+    } else {
+      const b = (await res.json().catch(() => null)) as {
+        error?: { message?: string };
+      } | null;
+      setError(b?.error?.message ?? '投稿に失敗しました');
+    }
+  }
+
+  return (
+    <form onSubmit={onSubmit} className="space-y-2">
+      <textarea
+        placeholder="コメント（Markdown）"
+        value={bodyMd}
+        onChange={(e) => setBodyMd(e.target.value)}
+        className="min-h-[80px] w-full rounded border px-3 py-2"
+        required
+      />
+      {error && (
+        <p className="text-sm text-red-600" role="alert">
+          {error}
+        </p>
+      )}
+      <button
+        type="submit"
+        disabled={loading}
+        className="rounded bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-700 disabled:opacity-50"
+      >
+        {loading ? '投稿中...' : 'コメント投稿'}
+      </button>
+    </form>
+  );
+}

components/board/MarkdownBody.tsx

@@ -0,0 +1,20 @@
+import ReactMarkdown from 'react-markdown';
+import remarkGfm from 'remark-gfm';
+import rehypeSanitize from 'rehype-sanitize';
+
+/**
+ * Markdown本文を安全にレンダリングする。
+ * HTML直接入力は無効化し、rehype-sanitize でサニタイズする。
+ */
+export function MarkdownBody({ bodyMd }: { bodyMd: string }) {
+  return (
+    <div className="prose prose-sm max-w-none">
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm]}
+        rehypePlugins={[rehypeSanitize]}
+      >
+        {bodyMd}
+      </ReactMarkdown>
+    </div>
+  );
+}

components/board/ThreadForm.tsx

@@ -0,0 +1,99 @@
+'use client';
+
+import { useState, type FormEvent } from 'react';
+import { useRouter } from 'next/navigation';
+
+const CATEGORIES = [
+  { value: '', label: 'なし' },
+  { value: 'notice', label: 'notice' },
+  { value: 'spec', label: 'spec' },
+  { value: 'minutes', label: 'minutes' },
+  { value: 'question', label: 'question' },
+  { value: 'decision', label: 'decision' },
+  { value: 'trouble', label: 'trouble' },
+  { value: 'memo', label: 'memo' },
+] as const;
+
+export function ThreadForm({ projectId }: { projectId: number }) {
+  const router = useRouter();
+  const [title, setTitle] = useState('');
+  const [bodyMd, setBodyMd] = useState('');
+  const [category, setCategory] = useState('');
+  const [error, setError] = useState<string | null>(null);
+  const [loading, setLoading] = useState(false);
+
+  async function onSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    setLoading(true);
+    setError(null);
+    const res = await fetch(`/api/projects/${projectId}/board/threads`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        title,
+        bodyMd,
+        category: category || undefined,
+      }),
+    });
+    setLoading(false);
+    if (res.ok) {
+      const data = (await res.json()) as { thread: { id: number } };
+      router.push(`/projects/${projectId}/board/${data.thread.id}`);
+      router.refresh();
+    } else {
+      const b = (await res.json().catch(() => null)) as {
+        error?: { message?: string };
+      } | null;
+      setError(b?.error?.message ?? '作成に失敗しました');
+    }
+  }
+
+  return (
+    <form
+      onSubmit={onSubmit}
+      className="space-y-3 rounded-lg border bg-white p-4 shadow-sm"
+    >
+      <div className="flex gap-2">
+        <input
+          type="text"
+          placeholder="タイトル"
+          value={title}
+          onChange={(e) => setTitle(e.target.value)}
+          className="flex-1 rounded border px-3 py-2"
+          required
+          maxLength={200}
+        />
+        <select
+          value={category}
+          onChange={(e) => setCategory(e.target.value)}
+          className="rounded border px-2 py-2"
+        >
+          {CATEGORIES.map((c) => (
+            <option key={c.value} value={c.value}>
+              {c.label}
+            </option>
+          ))}
+        </select>
+      </div>
+      <textarea
+        placeholder="本文（Markdown）"
+        value={bodyMd}
+        onChange={(e) => setBodyMd(e.target.value)}
+        className="min-h-[120px] w-full rounded border px-3 py-2"
+        required
+      />
+      {error && (
+        <p className="text-sm text-red-600" role="alert">
+          {error}
+        </p>
+      )}
+      <button
+        type="submit"
+        disabled={loading}
+        className="rounded bg-blue-600 px-4 py-2 font-medium text-white hover:bg-blue-700 disabled:opacity-50"
+      >
+        {loading ? '作成中...' : 'スレッド作成'}
+      </button>
+    </form>
+  );
+}