chore: add WeasyPrint system deps to Dockerfile python stage

apt-get install libcairo2 libpango-1.0-0 libpangoft2-1.0-0 libharfbuzz0b
libfontconfig1 shared-mime-info — minimal set for text-only PDF rendering.
libgdk-pixbuf-2.0-0 excluded (no images in PDF, verified via weasyprint --info).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/Dockerfile

@@ -9,6 +9,14 @@ RUN npm run build
 # Stage 2: python runtime
 FROM python:3.12-slim
 WORKDIR /app
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libcairo2 \
+    libpango-1.0-0 \
+    libpangoft2-1.0-0 \
+    libharfbuzz0b \
+    libfontconfig1 \
+    shared-mime-info \
+    && rm -rf /var/lib/apt/lists/*
 COPY backend/requirements.txt ./backend/
 RUN pip install --no-cache-dir -r backend/requirements.txt
 COPY backend/ ./backend/