Image Watermark API with Text Overlay

POST /upload - Upload image with text watermark
  Parameters:
  - image: File (JPG/PNG)
  - size: sm|md|lg|original (default: md)
  - watermark: Text for watermark (default: 'Sample Watermark')
  - opacity: Watermark opacity 0.1-1.0 (default: 0.5)
  - pattern: single|tiled (default: tiled)

Returns an HTML page with the image and CSS-based text watermark overlay.