feat: add Stirling PDF tool to Kit stack

Added Stirling PDF (S-PDF) to the Kit toolkit at sterling.kit.pivoine.art:
- PDF manipulation tool with 100+ features
- Merge, split, convert, OCR, compress PDFs
- Privacy-focused: local processing, no data collection
- Image: frooodle/s-pdf:latest
- Exposed on port 8080 via Traefik with SSL

Configuration:
- Added sterling service to kit/compose.yaml
- Added KIT_STERLING_IMAGE and KIT_STERLING_TRAEFIK_HOST to arty.yml
- Follows same pattern as other Kit tools (Vert, Paint, Pastel)
- Watchtower enabled for automatic updates

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

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

arty.yml

@@ -121,6 +121,8 @@ envs:
     KIT_PASTEL_API_IMAGE: ghcr.io/valknarness/pastel-api:latest
     KIT_PASTEL_UI_IMAGE: ghcr.io/valknarness/pastel-ui:latest
     KIT_PASTEL_TRAEFIK_HOST: pastel.kit.pivoine.art
+    KIT_STERLING_IMAGE: frooodle/s-pdf:latest
+    KIT_STERLING_TRAEFIK_HOST: sterling.kit.pivoine.art
     # Jellyfin
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly