feat: add HTTP auth and Sablier scale-to-zero to VERT stack

Added authentication and scale-to-zero capabilities to VERT file converter service:

**Authentication**:
- Added HTTP Basic Auth middleware using VERT_AUTH_USERS
- Auth middleware applied to web-secure router
- Credentials configured via .env file (htpasswd format)

**Sablier Scale-to-Zero**:
- Added sablier.enable and sablier.group labels
- Created proxy/dynamic/vert-sablier.yaml with Sablier middleware config
- 1-hour session duration before automatic scale-down
- Ghost theme with custom display name
- Middleware chain: sablier-vert@file → auth → compress

**Configuration Updates**:
- Added VERT_SABLIER_ENABLED to arty.yml (default: true)
- Updated CLAUDE.md with auth and Sablier documentation
- Middleware order ensures Sablier wakes container before auth check

Infrastructure:
- Follows same pattern as Scrapy service for auth
- Dynamic Traefik configuration for Sablier plugin
- Container name: vert_app (referenced in Sablier config)

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

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

arty.yml

@@ -115,6 +115,7 @@ envs:
     VERT_COMPOSE_PROJECT_NAME: vert
     VERT_IMAGE: ghcr.io/vert-sh/vert:latest
     VERT_TRAEFIK_HOST: vert.pivoine.art
+    VERT_SABLIER_ENABLED: true
     # Proxy
     PROXY_COMPOSE_PROJECT_NAME: proxy
     PROXY_DOCKER_IMAGE: traefik:latest