feat: add Pastel color palette generator to Kit stack

Added Pastel service with API and UI to the Kit toolkit:

**New Services:**
- pastel_api: Backend API for color palette generation
  - Image: ghcr.io/valknarness/pastel-api:latest
  - Routes: https://pastel.kit.pivoine.art/api

- pastel_ui: Frontend UI for interactive palette generation
  - Image: ghcr.io/valknarness/pastel-ui:latest
  - Routes: https://pastel.kit.pivoine.art

**Features:**
- Color harmony algorithms
- Interactive palette generation
- Export in various formats
- Programmatic API access
- Path-based routing (UI on root, API on /api)

**Configuration:**
- Updated arty.yml with KIT_PASTEL_* variables
- Updated documentation (CLAUDE.md, README.md)
- Added Traefik labels with SSL, compression, security headers
- Watchtower auto-update enabled

Kit stack now includes 5 services:
- Landing page (kit.pivoine.art)
- Vert file converter (vert.kit.pivoine.art)
- Paint image editor (paint.kit.pivoine.art)
- Pastel color generator (pastel.kit.pivoine.art)

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

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

CLAUDE.md

@@ -251,10 +251,11 @@ Joplin Server note-taking and synchronization platform:
 5. Enter email and password created in step 1
 
 ### Kit (kit/compose.yaml)
-Unified toolkit with landing page, file conversion, and image editing using subdomain routing:
+Unified toolkit with landing page, file conversion, image editing, and color palette generation using subdomain routing:
 - **Landing**: `kit.pivoine.art` - Toolkit landing page
 - **Vert**: `vert.kit.pivoine.art` - Universal file format converter
 - **Paint**: `paint.kit.pivoine.art` - Web-based image editor
+- **Pastel**: `pastel.kit.pivoine.art` - Color palette generator (API + UI)
 
 #### Landing Page (kit.pivoine.art)
 Kit toolkit landing page:
@@ -297,7 +298,31 @@ miniPaint web-based image editor built from GitHub:
 **Usage**:
 Access https://paint.kit.pivoine.art to use the image editor. All editing happens in the browser - images are not uploaded to the server.
 
-**Note**: Both Kit services are stateless and don't require backups as no data is persisted.
+#### Pastel Service (pastel.kit.pivoine.art)
+Pastel color palette generator with API and UI:
+- Generate beautiful color palettes
+- API endpoint at `/api` for programmatic access
+- Web UI for interactive palette generation
+- Color harmony algorithms
+- Export palettes in various formats
+- Stateless architecture
+
+**Architecture**:
+- **API**: Backend service handling color generation logic
+- **UI**: Frontend application consuming the API
+
+**Images**:
+- API: `ghcr.io/valknarness/pastel-api:latest`
+- UI: `ghcr.io/valknarness/pastel-ui:latest`
+
+**Routing**:
+- UI: `https://pastel.kit.pivoine.art` (root path)
+- API: `https://pastel.kit.pivoine.art/api` (path prefix)
+
+**Usage**:
+Access https://pastel.kit.pivoine.art to generate and explore color palettes interactively.
+
+**Note**: Kit services (Vert, Paint, Pastel) are stateless and don't require backups as no data is persisted.
 
 ### PairDrop (drop/compose.yaml)
 PairDrop peer-to-peer file sharing service: