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.

.github/workflows/backend.yml

@@ -27,15 +27,6 @@ jobs:
           --health-interval 10s
           --health-timeout 5s
           --health-retries 5
-      redis:
-        image: redis:7-alpine
-        ports:
-          - 6379:6379
-        options: >-
-          --health-cmd "redis-cli ping"
-          --health-interval 10s
-          --health-timeout 5s
-          --health-retries 5
     env:
       SECRET_KEY: github-ci-secret-key
       DEBUG: "False"
@@ -83,8 +74,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r requirements.txt
-          pip install coverage
+          pip install -r requirements-test.txt
 
       - name: Prepare environment file
         run: cp .env.test .env
@@ -92,10 +82,13 @@ jobs:
       - name: Run migrations
         run: python manage.py migrate --noinput
 
-      - name: Run tests with coverage
+      - name: Run unit and integration tests with coverage
         run: |
-          coverage run --rcfile=.coveragerc manage.py test apps.users.tests apps.events.tests apps.payments.tests apps.notifications.tests apps.communications.tests core.tests --settings=config.settings.test --verbosity 2
+          coverage run --rcfile=.coveragerc -m pytest -m unit -q
+          coverage run --rcfile=.coveragerc --append -m pytest -m integration -q
           coverage report -m
+          coverage json -o coverage.json
+          python scripts/check_coverage.py coverage.json --minimum 80
 
   deploy:
     runs-on: ubuntu-latest