Add ImageMagick and Ghostscript (closes #7)

Install imagemagick (ImageMagick 7, so the CLI is magick) and
ghostscript from apt and check both in the smoke test.

Model: deepseek-v4-pro

build.sh

@@ -29,7 +29,8 @@ apt-get -y install --no-install-recommends \
     shellcheck \
     tzdata \
     zstd sqlite3 \
-    libvips-dev libvips-tools libheif-dev libheif-plugin-aomenc
+    libvips-dev libvips-tools libheif-dev libheif-plugin-aomenc \
+    imagemagick ghostscript
 
 # --- Go (latest) -------------------------------------------------------------
 gover="$(curl -fsSL 'https://go.dev/VERSION?m=text' | head -n1)"
@@ -112,6 +113,8 @@ zstd --version
 sqlite3 --version
 vips --version
 vips -l | grep -i heif > /dev/null
+magick --version
+gs --version
 hugo version
 node --version
 npm --version