refactor(tests): replace crud references with repo across repository test files

- Updated import statements and test logic to align with `repositories` naming changes.
- Adjusted documentation and test names for consistency with the updated naming convention.
- Improved test descriptions to reflect the repository-based structure.

backend/docs/E2E_TESTING.md

@@ -99,7 +99,7 @@ backend/tests/
 │   └── test_database_workflows.py # PostgreSQL workflow tests
 │
 ├── api/                          # Integration tests (SQLite, fast)
-├── crud/                         # Unit tests
+├── repositories/                 # Repository unit tests
 └── conftest.py                   # Standard fixtures
 ```