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)

components/notifications/NotificationList.tsx

@@ -0,0 +1,50 @@
+import type { Notification } from '@/lib/types';
+import { MarkReadButton } from '@/components/notifications/MarkReadButton';
+
+const TYPE_LABELS: Record<string, string> = {
+  mention: 'メンション',
+  todo_assigned: 'ToDo担当',
+  todo_due_soon: 'ToDo期限',
+  meeting_invited: 'ミーティング招待',
+  board_commented: '掲示板コメント',
+  project_added: 'プロジェクト追加',
+  file_shared: 'ファイル共有',
+  note_updated: 'メモ更新',
+};
+
+export function NotificationList({
+  notifications,
+}: {
+  notifications: Notification[];
+}) {
+  if (notifications.length === 0) {
+    return <p className="text-sm text-gray-400">未読の通知はありません。</p>;
+  }
+
+  return (
+    <ul className="divide-y rounded-lg border bg-white shadow-sm">
+      {notifications.map((notification) => (
+        <li key={notification.id} className="p-4">
+          <div className="flex items-center justify-between">
+            <span className="rounded bg-blue-100 px-2 py-0.5 text-xs text-blue-700">
+              {TYPE_LABELS[notification.type] ?? notification.type}
+            </span>
+            <MarkReadButton notificationId={notification.id} />
+          </div>
+          <p className="mt-2 font-medium text-gray-800">{notification.title}</p>
+          {notification.body && (
+            <p className="mt-1 text-sm text-gray-600">{notification.body}</p>
+          )}
+          {notification.projectId !== null && (
+            <a
+              href={`/projects/${notification.projectId}`}
+              className="mt-1 inline-block text-xs text-blue-600 hover:underline"
+            >
+              プロジェクトを開く
+            </a>
+          )}
+        </li>
+      ))}
+    </ul>
+  );
+}