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>

backend/.env.example

@@ -20,6 +20,8 @@ DEBUG=false
 TAVILY_API_KEY=
 # 蓄積知識をシステムプロンプトに注入する際の最大件数
 KNOWLEDGE_CONTEXT_LIMIT=15
+# Web検索結果を構造的な要約にまとめる際に使うモデル（未設定ならDEFAULT_MODELを使用）
+SEARCH_SUMMARY_MODEL=
 
 # ローカルMarkdownナレッジベース（backend/knowledge_base/ 配下のファイルを話題に応じて注入）
 LOCAL_KB_DIR=knowledge_base