Update backend requirements - security hardening

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.12-slim AS base
+FROM python:3.13-slim AS base
 
 ENV PYTHONDONTWRITEBYTECODE=1
 ENV PYTHONUNBUFFERED=1
@@ -6,6 +6,10 @@ ENV PIP_NO_CACHE_DIR=1
 
 WORKDIR /app
 
+RUN apt-get update \
+    && apt-get upgrade -y \
+    && rm -rf /var/lib/apt/lists/*
+
 RUN addgroup --system app && adduser --system --ingroup app app
 
 COPY requirements.txt /app/requirements.txt