Move image pixel reads to get_flattened_data and add the font libraries

Pillow 12 renames Image.getdata to get_flattened_data; the award image
normaliser and the isslop hue histogram both read pixels that way. The image
stack also needs pango, harfbuzz, fontconfig and a base font in the container,
so text rendering has glyphs to work with.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -4,6 +4,8 @@ WORKDIR /app
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     curl ca-certificates \
+    libglib2.0-0 libpango-1.0-0 libpangoft2-1.0-0 libharfbuzz0b libfontconfig1 \
+    fonts-dejavu-core \
     && rm -rf /var/lib/apt/lists/*
 
 # Optional: the docker CLI so the (admin-only) container manager can drive the host