feat: add VERT file converter stack

Added VERT universal file format converter deployed at vert.pivoine.art:

**Vert stack** (vert.pivoine.art):
- vert: WebAssembly-based file converter
  - Supports 250+ file formats (images, audio, documents, video)
  - Client-side processing for privacy
  - No file size limits
  - No persistent data storage (stateless)

Infrastructure updates:
- Created vert/compose.yaml with Vert configuration
- Added VERT_* environment variables to arty.yml
- Updated compose.yaml to include vert stack
- Updated README.md and CLAUDE.md documentation
- No backup needed (stateless service)

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

@@ -110,6 +110,11 @@ envs:
     JOPLIN_TRAEFIK_HOST: joplin.pivoine.art
     JOPLIN_APP_PORT: 22300
     JOPLIN_DB_NAME: joplin
+    # Vert
+    VERT_TRAEFIK_ENABLED: true
+    VERT_COMPOSE_PROJECT_NAME: vert
+    VERT_IMAGE: ghcr.io/vert-sh/vert:latest
+    VERT_TRAEFIK_HOST: vert.pivoine.art
     # Proxy
     PROXY_COMPOSE_PROJECT_NAME: proxy
     PROXY_DOCKER_IMAGE: traefik:latest