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/gallery/tests/integration/test_tasks.py

@@ -0,0 +1,28 @@
+import pytest
+from django.core.files.base import ContentFile
+
+from apps.gallery.models import Gallery
+from apps.gallery.tasks import process_uploaded_image
+
+
+pytestmark = [pytest.mark.integration, pytest.mark.django_db]
+
+
+def test_gallery_processing_updates_metadata(user_factory, tmp_path, settings, mocker):
+    settings.MEDIA_ROOT = str(tmp_path)
+    gallery = Gallery(title="Photo", uploaded_by=user_factory())
+    gallery._defer_image_processing = True
+    gallery.image.save("gallery/photo.jpg", ContentFile(b"image data"))
+    result = mocker.Mock(file_size=10, width=20, height=30)
+    process = mocker.patch("apps.gallery.tasks.safe_process_public_image", return_value=result)
+    assert process_uploaded_image.run(gallery.pk) == "Processed image: Photo"
+    gallery.refresh_from_db()
+    assert (gallery.file_size, gallery.width, gallery.height) == (10, 20, 30)
+    process.assert_called_once()
+
+
+def test_gallery_processing_missing_image_and_id(user_factory):
+    empty = Gallery.objects.create(title="Empty", uploaded_by=user_factory())
+    assert process_uploaded_image.run(empty.pk) is None
+    with pytest.raises(Gallery.DoesNotExist):
+        process_uploaded_image.run(999999)