fix(infra) : add qpdf to production Docker image

qpdf was only installed in dev Dockerfile, causing the PDF compressor
to silently skip compression in production.

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

infra/prod/Dockerfile

@@ -42,7 +42,7 @@ FROM php:8.4-fpm AS production
 
 RUN apt-get update && apt-get install -y \
         libicu-dev libpq-dev libpng-dev libzip-dev libxml2-dev \
-        nginx supervisor \
+        nginx supervisor qpdf \
     && docker-php-ext-install -j$(nproc) intl pdo_pgsql zip gd opcache \
     && rm -rf /var/lib/apt/lists/*