Add image generation API endpoint

[Copilot]

Adds programmatic placeholder image generation for products and categories.

Changes

• server/lib/image_generator.cjs - SVG-based image generator using Sharp (existing dep)

  • Customizable dimensions (1-4000px), text, colors, format (JPEG/PNG)
  • Returns API-compatible image paths

• server/api/routes_image_generator.cjs - POST /api/v1/generate-image endpoint

  • Input validation with error responses
  • Authenticated via existing x-stjorna-userid header

• server/test/api/image_generator.spec.cjs - 5 tests covering success/error cases

• Documentation - API usage in server/README.md, feature list in root README.md

Usage

POST /api/v1/generate-image
{
  "width": 400,
  "height": 300,
  "text": "Product Name",
  "backgroundColor": "#3498db",
  "textColor": "#ffffff",
  "format": "png"
}

# Returns: { "imageUrl": "/data/uploads/userid/products/hash.png", ... }

Use cases: placeholder generation during product creation, bulk imports, category thumbnails.

Original prompt

can you generate images?

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