fix: use correct package name for gdk-pixbuf in Debian Trixie

libgdk-pixbuf2.0-dev was renamed to libgdk-pixbuf-2.0-dev in Trixie
(python:3.12-slim base image).

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

backend/Dockerfile

@@ -8,7 +8,7 @@ RUN apt-get update && apt-get install -y \
     libpq-dev \
     libpango1.0-dev \
     libcairo2-dev \
-    libgdk-pixbuf2.0-dev \
+    libgdk-pixbuf-2.0-dev \
     libffi-dev \
     && rm -rf /var/lib/apt/lists/*