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.

config/tests/integration/test_routing.py

@@ -0,0 +1,9 @@
+import pytest
+
+
+pytestmark = [pytest.mark.integration, pytest.mark.django_db]
+
+
+def test_api_routes_are_wired(client):
+    assert client.get("/api/health").status_code == 200
+    assert client.get("/api/docs").status_code == 200

config/tests/unit/test_environment.py

@@ -0,0 +1,15 @@
+from types import SimpleNamespace
+
+import pytest
+
+from config.services.unfold import environment_callback
+
+
+pytestmark = pytest.mark.unit
+
+
+def test_admin_environment_badge(settings):
+    settings.DEBUG = True
+    assert environment_callback(SimpleNamespace()) == ["Development", "warning"]
+    settings.DEBUG = False
+    assert environment_callback(SimpleNamespace()) == ["Production", "success"]