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>

docker-compose.yml

@@ -0,0 +1,28 @@
+services:
+  backend:
+    build:
+      context: ./backend
+    restart: unless-stopped
+    env_file:
+      - ./backend/.env
+    environment:
+      # SQLite ファイルと知識ベースを永続ボリュームに置く（.env の値を上書き）
+      DATABASE_URL: sqlite:////app/data/playchat.db
+    volumes:
+      - backend_data:/app/data
+      - backend_knowledge_base:/app/knowledge_base
+    expose:
+      - "8000"
+
+  frontend:
+    build:
+      context: ./frontend
+    restart: unless-stopped
+    depends_on:
+      - backend
+    ports:
+      - "80:80"
+
+volumes:
+  backend_data:
+  backend_knowledge_base: