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

portal/tests/conftest.py

@@ -0,0 +1,10 @@
+import pytest
+
+
+@pytest.fixture
+def valid_env(tmp_path):
+    return {
+        "SECRET_KEY": "x" * 48,
+        "ALLOWED_HOSTS": "testserver",
+        "DATA_DIR": str(tmp_path),
+    }