feat: add Django project scaffold, configuration and container

Environment-variable configuration that fails fast, database-only health
check, JSON request logs with a request id, security headers with a strict
CSP, and a multi-stage non-root Docker image with compose file. Ticket 1.1.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sk72AxLwQnYttF6VFSxQLt

docker/entrypoint.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -eu
+
+python manage.py migrate --noinput
+
+exec gunicorn portal.config.wsgi:application \
+    --bind 0.0.0.0:8000 \
+    --workers 1 \
+    --threads 4 \
+    --preload \
+    --worker-tmp-dir /tmp \
+    --error-logfile -