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/notifications/page.tsx

@@ -0,0 +1,27 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser, toPublicUser } from '@/lib/auth/getCurrentUser';
+import { createNotificationService } from '@/lib/api/services';
+import { Header } from '@/components/layout/Header';
+import { NotificationList } from '@/components/notifications/NotificationList';
+
+export const dynamic = 'force-dynamic';
+
+export default async function NotificationsPage() {
+  const user = await getCurrentUser();
+  if (!user) {
+    redirect('/login');
+  }
+
+  const service = createNotificationService();
+  const { items } = service.listUnread(user.id, 1);
+
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <Header user={toPublicUser(user)} />
+      <main className="mx-auto max-w-2xl space-y-6 p-6">
+        <h1 className="text-2xl font-bold">通知</h1>
+        <NotificationList notifications={items} />
+      </main>
+    </div>
+  );
+}