Add Docker/Lightsail deployment, web search summarization, and fix vite peer dep

- Add backend/frontend Dockerfiles, nginx reverse proxy config, and
  docker-compose.yml to run the app on AWS Lightsail; document the setup
  in docs/deploy-lightsail.md. Fix a .gitignore path anchoring bug that
  left backend/knowledge_base/web_search/ untracked instead of ignored.
- Summarize web search results into structured Markdown via LLM before
  saving them to the local knowledge base, and support regenerating
  existing entries from their source URLs (backend/app/core/memory_kb).
- Downgrade vite to ^6 to match @vitejs/plugin-vue's peer dependency
  range, fixing a failing `npm ci` (found while testing the Docker build).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -10,6 +10,7 @@ build/
 .venv/
 venv/
 .uv/
+backend/knowledge_base/web_search/
 
 # 環境変数・シークレット
 backend/.env