feat: add miniPaint stack (paint.pivoine.art)

Added new paint service stack to the docker-compose infrastructure:

- **Paint stack** (paint.pivoine.art):
  - miniPaint: Web-based image editor built from GitHub
  - Multi-stage Docker build clones from https://github.com/viliusle/miniPaint
  - Features: layers, filters, drawing tools, text, shapes support
  - Client-side processing with no server uploads
  - Stateless architecture (no backups needed)

Infrastructure updates:
- Created paint/compose.yaml with Traefik routing and SSL
- Created paint/Dockerfile with Node.js build stage and nginx serve
- Added PAINT environment variables to arty.yml
- Updated compose.yaml include list
- Updated CLAUDE.md documentation

All services integrated with Traefik for SSL termination and include
Watchtower auto-update labels.

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

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

compose.yaml

@@ -11,6 +11,7 @@ include:
   - vault/compose.yaml
   - joplin/compose.yaml
   - vert/compose.yaml
+  - paint/compose.yaml
   - restic/compose.yaml
   - umami/compose.yaml
   - sablier/compose.yaml