refactor(ai): remove crawl4ai service, add backrest config to repo

- Remove crawl4ai service from ai/compose.yaml (will use local MCP instead)
- Remove crawl4ai backup volume from core/compose.yaml
- Add core/backrest/config.json (infrastructure as code)
- Change backrest from volume to bind-mounted config
- Update CLAUDE.md and README.md documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

arty.yml

@@ -135,7 +135,6 @@ envs:
     AI_COMPOSE_PROJECT_NAME: ai
     AI_POSTGRES_IMAGE: pgvector/pgvector:pg16
     AI_WEBUI_IMAGE: ghcr.io/open-webui/open-webui:main
-    AI_CRAWL4AI_IMAGE: unclecode/crawl4ai:latest
     AI_FACEFUSION_IMAGE: facefusion/facefusion:3.5.0-cpu
     AI_FACEFUSION_TRAEFIK_ENABLED: true
     AI_FACEFUSION_TRAEFIK_HOST: facefusion.ai.pivoine.art