feat(m5): notifications & activity log foundation

- Expand NotificationRepository (unread list paginated, count, markRead
  ownership-scoped); new ActivityLogRepository (create, findByProject
  paginated) with deterministic created_at+id ordering
- NotificationService (notifyOnEvent, resolveTargets for 8 event types,
  injectable SseBroadcaster for M8) + ActivityLogService (logActivity,
  listByProject)
- NotificationEvent/SseEvent/SseBroadcaster types in lib/types
- APIs: GET /api/notifications, POST /api/notifications/:id/read,
  GET /api/projects/:projectId/activity
- Screens: notifications list + MarkReadButton, NotificationBadge in
  Header, project activity page + ProjectNav link
- Unit tests for both repositories and services (28 new)

app/projects/[projectId]/activity/page.tsx

@@ -0,0 +1,81 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser, toPublicUser } from '@/lib/auth/getCurrentUser';
+import {
+  createProjectService,
+  createActivityLogService,
+} from '@/lib/api/services';
+import { Header } from '@/components/layout/Header';
+import { ProjectNav } from '@/components/layout/ProjectNav';
+import { ForbiddenError, NotFoundError } from '@/lib/errors';
+
+export const dynamic = 'force-dynamic';
+
+const ACTION_LABELS: Record<string, string> = {
+  todo_created: 'ToDo作成',
+  todo_updated: 'ToDo更新',
+  todo_completed: 'ToDo完了',
+  file_uploaded: 'ファイルアップロード',
+  board_posted: '掲示板投稿',
+  comment_added: 'コメント追加',
+  note_created: 'メモ作成',
+  note_updated: 'メモ更新',
+  meeting_created: 'ミーティング作成',
+  member_added: 'メンバー追加',
+  milestone_updated: 'マイルストーン更新',
+};
+
+export default async function ProjectActivityPage({
+  params,
+}: {
+  params: Promise<{ projectId: string }>;
+}) {
+  const user = await getCurrentUser();
+  if (!user) {
+    redirect('/login');
+  }
+  const { projectId } = await params;
+
+  const projectService = createProjectService();
+  let project;
+  try {
+    project = projectService.getProject(user.id, Number(projectId));
+  } catch (error) {
+    if (error instanceof ForbiddenError || error instanceof NotFoundError) {
+      redirect('/dashboard');
+    }
+    throw error;
+  }
+
+  const activityService = createActivityLogService();
+  const { items } = activityService.listByProject(project.id, 1);
+
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <Header user={toPublicUser(user)} />
+      <ProjectNav projectId={project.id} active="activity" />
+      <main className="mx-auto max-w-3xl space-y-6 p-6">
+        <h1 className="text-2xl font-bold">アクティビティログ</h1>
+        {items.length === 0 ? (
+          <p className="text-sm text-gray-400">アクティビティはありません。</p>
+        ) : (
+          <ul className="divide-y rounded-lg border bg-white shadow-sm">
+            {items.map((log) => (
+              <li key={log.id} className="p-4">
+                <div className="flex items-center justify-between">
+                  <span className="rounded bg-gray-100 px-2 py-0.5 text-xs">
+                    {ACTION_LABELS[log.action] ?? log.action}
+                  </span>
+                  <span className="text-xs text-gray-400">{log.createdAt}</span>
+                </div>
+                <p className="mt-2 text-sm text-gray-700">
+                  {log.targetType}
+                  {log.targetId !== null ? ` #${log.targetId}` : ''}
+                </p>
+              </li>
+            ))}
+          </ul>
+        )}
+      </main>
+    </div>
+  );
+}