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>

README.md

@@ -54,6 +54,7 @@ The **Falcon** is a state-of-the-art containerized starship, powered by Docker's
 | **LINKS** | *Interstellar bookmark archive* | [links.pivoine.art](https://links.pivoine.art) |
 | **VAULT** | *Encrypted password vault* | [vault.pivoine.art](https://vault.pivoine.art) |
 | **JOPLIN** | *Note-taking server & sync hub* | [joplin.pivoine.art](https://joplin.pivoine.art) |
+| **VERT** | *Universal file format converter* | [vert.pivoine.art](https://vert.pivoine.art) |
 | **RESTIC** | *Automated backup vault system* | [restic.pivoine.art](https://restic.pivoine.art) |
 | **PROXY** | *Shield control dashboard* | [proxy.pivoine.art](https://proxy.pivoine.art) |
 | **VPN** | *Cloaking device network* | [vpn.pivoine.art](https://vpn.pivoine.art) |
@@ -209,6 +210,7 @@ THE FALCON (falcon_network)
 │  ├─ Linkwarden Marks   [links.pivoine.art]
 │  ├─ Vaultwarden Vault  [vault.pivoine.art]
 │  ├─ Joplin Sync Server [joplin.pivoine.art]
+│  ├─ Vert Converter     [vert.pivoine.art]
 │  ├─ Backrest Backups   [restic.pivoine.art]
 │  └─ WireGuard VPN      [vpn.pivoine.art]
 │