feat: add initial Docker Compose infrastructure

infra/docker/backend/Dockerfile

@@ -0,0 +1,22 @@
+FROM python:3.13.4-slim
+
+ENV PYTHONDONTWRITEBYTECODE=1
+ENV PYTHONUNBUFFERED=1
+
+WORKDIR /app
+
+RUN pip install --no-cache-dir \
+    "Django==5.2.3" \
+    "djangorestframework==3.16.0" \
+    "gunicorn==23.0.0" \
+    "psycopg[binary]==3.2.9"
+
+RUN useradd --create-home --shell /usr/sbin/nologin appuser
+
+COPY placeholder_wsgi.py /app/placeholder_wsgi.py
+
+USER appuser
+
+EXPOSE 8000
+
+CMD ["gunicorn", "--bind", "0.0.0.0:8000", "placeholder_wsgi:application"]