Add interface design references and principles; implement sport service tests

- Introduced critique, example, principles, and validation documents for interface design.
- Enhanced backend service with coverage threshold and fixed variable declaration in sport service.
- Added comprehensive tests for create, update, and list sports functionalities.

apps/backend/bunfig.toml

@@ -1,2 +1,3 @@
 [test]
 preload = ["./test/support/prisma.mock.ts"]
+coverageThreshold = 0.8