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>
+  );
+}