feat: add AI stack with Open WebUI, Crawl4AI, and pgvector

Created complete AI infrastructure stack at ai.pivoine.art:

**New Services:**
- **Open WebUI** (ai.pivoine.art)
  - ChatGPT-like interface for AI models
  - Multi-user chat with authentication
  - RAG (Retrieval-Augmented Generation) support
  - Document upload and processing
  - Claude API integration via Anthropic

- **PostgreSQL with pgvector** (dedicated AI database)
  - Vector similarity search for RAG
  - Separate from production databases
  - Stores embeddings and documents

- **Crawl4AI** (internal API service)
  - Web scraping optimized for LLMs
  - Converts websites to clean Markdown
  - Called by n8n workflows
  - No public exposure (internal only)

**Configuration:**
- Added 18 AI environment variables to arty.yml
- Configured email notifications via IONOS SMTP
- OpenAI API compatibility for Claude integration
- Traefik SSL termination and compression

**Backup:**
- Added 3 AI volumes to Restic backup
- Daily backup at 3 AM
- Retention: 7 daily, 4 weekly, 6 monthly, 2 yearly

**Integration:**
- Shares falcon_network with existing services
- Ready for n8n workflow automation
- Mattermost notifications support
- Watchtower auto-updates enabled

Ready for Phase 2: GPU server integration with Ollama, Whisper, and
Stable Diffusion when IONOS A10 server is provisioned.

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

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

compose.yaml

@@ -14,6 +14,7 @@ include:
   - kit/compose.yaml
   - jelly/compose.yaml
   - drop/compose.yaml
+  - ai/compose.yaml
   - restic/compose.yaml
   - netdata/compose.yaml
   - umami/compose.yaml