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.

apps/blog/tests/unit/test_admin_actions.py

@@ -0,0 +1,33 @@
+from types import SimpleNamespace
+
+import pytest
+from django.contrib import admin
+
+from apps.blog.admin import CommentAdmin, PostAdmin
+from apps.blog.models import Comment, Post
+
+
+pytestmark = pytest.mark.unit
+
+
+def test_blog_admin_preview_shortens_long_comment():
+    model_admin = CommentAdmin(Comment, admin.site)
+    assert model_admin.content_preview(SimpleNamespace(content="short")) == "short"
+    assert model_admin.content_preview(SimpleNamespace(content="x" * 51)) == "x" * 50 + "..."
+
+
+def test_post_admin_actions_update_status_and_featured_flag(mocker):
+    model_admin = PostAdmin(Post, admin.site)
+    mocker.patch.object(model_admin, "message_user")
+    queryset = mocker.Mock()
+    queryset.count.return_value = 1
+    user = SimpleNamespace(id=1)
+    request = SimpleNamespace(user=user)
+    model_admin.submit_for_review(request, queryset)
+    queryset.update.assert_called_with(status="submitted")
+    model_admin.request_changes(request, queryset)
+    queryset.update.assert_called_with(status="changes_requested", reviewed_by=user)
+    model_admin.make_published(request, queryset)
+    queryset.update.assert_called_with(status="published", published_by=user)
+    model_admin.make_featured(request, queryset)
+    queryset.update.assert_called_with(is_featured=True)