Add imagemagick to CI dependencies (#15)

Lektor requires imagemagick for image processing during build.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: roberto-arista

.github/workflows/deploy.yml

@@ -16,6 +16,8 @@ jobs:
         with:
           lfs: true
 
+      - run: sudo apt-get update && sudo apt-get install -y imagemagick
+
       - uses: astral-sh/setup-uv@v5
 
       - run: uv sync