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>

arty.yml

@@ -116,6 +116,10 @@ envs:
     VERT_IMAGE: ghcr.io/vert-sh/vert:latest
     VERT_TRAEFIK_HOST: vert.pivoine.art
     VERT_SABLIER_ENABLED: true
+    # Paint
+    PAINT_TRAEFIK_ENABLED: true
+    PAINT_COMPOSE_PROJECT_NAME: paint
+    PAINT_TRAEFIK_HOST: paint.pivoine.art
     # Proxy
     PROXY_COMPOSE_PROJECT_NAME: proxy
     PROXY_DOCKER_IMAGE: traefik:latest