Refactor test utilities and enhance ASGI settings

- Cleaned up and standardized assertions in ApiTestMixin for API response validation.
- Updated ASGI settings to use os.environ for setting the DJANGO_SETTINGS_MODULE.
- Removed unused imports and improved formatting in settings.py.
- Refactored URL patterns in urls.py for better readability and organization.
- Enhanced view functions in views.py for consistency and clarity.
- Added .flake8 configuration for linting and style enforcement.
- Introduced type stubs for django-environ to improve type checking with Pylance.

.flake8

@@ -0,0 +1,29 @@
+[flake8]
+# Maximum line length (matches Black formatter)
+max-line-length = 88
+
+# Exclude common directories that shouldn't be linted
+exclude = 
+    .git,
+    __pycache__,
+    .venv,
+    venv,
+    env,
+    .env,
+    migrations,
+    node_modules,
+    .tox,
+    .mypy_cache,
+    .pytest_cache,
+    build,
+    dist,
+    *.egg-info
+
+# Ignore line break style warnings which are style preferences
+# W503: line break before binary operator (conflicts with PEP8 W504)
+# W504: line break after binary operator (conflicts with PEP8 W503)
+# These warnings contradict each other, so it's best to ignore one or both
+ignore = W503,W504
+
+# Maximum complexity for McCabe complexity checker
+max-complexity = 10