feat: add PDF export generation via WeasyPrint with branded template

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -6,6 +6,10 @@ WORKDIR /app
 RUN apt-get update && apt-get install -y \
     gcc \
     libpq-dev \
+    libpango1.0-dev \
+    libcairo2-dev \
+    libgdk-pixbuf2.0-dev \
+    libffi-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Python dependencies