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/MarkReadButton.tsx

@@ -0,0 +1,31 @@
+'use client';
+
+import { useState } from 'react';
+import { useRouter } from 'next/navigation';
+
+export function MarkReadButton({ notificationId }: { notificationId: number }) {
+  const router = useRouter();
+  const [busy, setBusy] = useState(false);
+
+  async function onRead() {
+    setBusy(true);
+    const res = await fetch(`/api/notifications/${notificationId}/read`, {
+      method: 'POST',
+    });
+    setBusy(false);
+    if (res.ok) {
+      router.refresh();
+    }
+  }
+
+  return (
+    <button
+      type="button"
+      onClick={onRead}
+      disabled={busy}
+      className="text-xs text-gray-500 hover:underline disabled:opacity-50"
+    >
+      {busy ? '処理中...' : '既読にする'}
+    </button>
+  );
+}

components/notifications/NotificationBadge.tsx

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+'use client';
+
+import { useEffect, useState } from 'react';
+
+export function NotificationBadge() {
+  const [count, setCount] = useState(0);
+
+  useEffect(() => {
+    let active = true;
+    fetch('/api/notifications?page=1')
+      .then((res) => (res.ok ? res.json() : null))
+      .then((data) => {
+        if (active && data && typeof data.total === 'number') {
+          setCount(data.total);
+        }
+      })
+      .catch(() => undefined);
+    return () => {
+      active = false;
+    };
+  }, []);
+
+  if (count === 0) {
+    return (
+      <a href="/notifications" className="text-gray-600 hover:underline">
+        通知
+      </a>
+    );
+  }
+
+  return (
+    <a
+      href="/notifications"
+      className="relative text-gray-600 hover:underline"
+      data-notification-count={count}
+    >
+      通知
+      <span className="ml-1 rounded-full bg-red-500 px-1.5 py-0.5 text-xs text-white">
+        {count > 99 ? '99+' : count}
+      </span>
+    </a>
+  );
+}

components/notifications/NotificationList.tsx

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+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>
+  );
+}