test: migrate backend suites to pytest with module coverage gate

Split backend tests into unit and integration suites, add isolated fixtures and coverage enforcement, and update CI and test documentation. Cover current authentication, content, notification, payment, media, and admin flows; fix behaviors revealed by the new tests. Validate both suites and the per-module gate against local PostgreSQL.

core/tests/integration/test_health.py

@@ -0,0 +1,10 @@
+import pytest
+
+
+pytestmark = [pytest.mark.integration, pytest.mark.django_db]
+
+
+def test_health_endpoint_reports_database_connection(client):
+    response = client.get("/api/health")
+    assert response.status_code == 200
+    assert response.json()["status"] == "ok"