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/api/schemas.py

@@ -17,9 +17,9 @@ class GallerySchema(ModelSchema):
     absolute_image_preview_url: Optional[str] = None
     absolute_image_blur_url: Optional[str] = None
 
-    class Config:
+    class Meta:
         model = Gallery
-        model_fields = ['id', 'title', 'description', 'image', 'alt_text',
+        fields = ['id', 'title', 'description', 'image', 'alt_text',
                         'width', 'height', 'is_public', 'created_at']
 
     @staticmethod