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/settings/test.py

@@ -1,3 +1,5 @@
+from tempfile import TemporaryDirectory
+
 from .base import *
 
 # Lightweight defaults keep local/CI test runs isolated from production infra.
@@ -31,6 +33,9 @@ CACHES = {
 CELERY_TASK_ALWAYS_EAGER = True
 CELERY_TASK_EAGER_PROPAGATES = True
 
+_test_static_root = TemporaryDirectory(prefix="backend-test-static-")
+STATIC_ROOT = _test_static_root.name
+
 # Tests should not enforce HTTPS-only cookies to simplify client simulations.
 CSRF_COOKIE_SECURE = False
 SESSION_COOKIE_SECURE = False