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/test/sport/create-sport.handler.test.ts

@@ -0,0 +1,97 @@
+import { beforeEach, expect, mock, test } from 'bun:test';
+
+import { Errors } from '@/lib/errors';
+import { err, ok } from '@/lib/result';
+import type { CreateSportInput } from '@repo/api-contract';
+
+const createSportMock = mock(async (_input: CreateSportInput) =>
+  ok({
+    id: 'sport-1',
+    name: 'Tenis',
+    slug: 'tenis',
+    isActive: true,
+    createdAt: '2026-04-23T00:00:00.000Z',
+    updatedAt: '2026-04-23T00:00:00.000Z',
+  })
+);
+
+mock.module('@/modules/sport/services/sport.service', () => ({
+  __esModule: true,
+  createSport: createSportMock,
+}));
+
+const { createSportHandler } = await import(
+  '@/modules/sport/handlers/create-sport.handler'
+);
+
+type HandlerContext = {
+  get: (key: 'requestId') => string | undefined;
+  req: {
+    valid: () => unknown;
+  };
+  json: ReturnType<typeof mock>;
+};
+
+function createContext(body: unknown) {
+  const json = mock((payload: unknown, init?: unknown) => ({
+    payload,
+    init,
+  }));
+
+  const c = {
+    get: () => undefined,
+    req: {
+      valid: () => body,
+    },
+    json,
+  } as unknown as HandlerContext;
+
+  return { c, json };
+}
+
+beforeEach(() => {
+  createSportMock.mockReset();
+  mock.clearAllMocks();
+});
+
+test('returns 201 with the created sport', async () => {
+  const createdSport = {
+    id: 'sport-1',
+    name: 'Tenis',
+    slug: 'tenis',
+    isActive: true,
+    createdAt: '2026-04-23T00:00:00.000Z',
+    updatedAt: '2026-04-23T00:00:00.000Z',
+  } as const;
+
+  createSportMock.mockResolvedValue(ok(createdSport));
+
+  const payload: CreateSportInput = { name: '  Tenis  ' };
+  const { c, json } = createContext(payload);
+
+  const response = await createSportHandler(c as never);
+
+  expect(createSportMock).toHaveBeenCalledWith(payload);
+  expect(json.mock.calls[0]?.[0]).toEqual(createdSport);
+  expect(json.mock.calls[0]?.[1]).toBe(201);
+  expect(response).toBeDefined();
+});
+
+test('returns a conflict problem when the service rejects with a conflict error', async () => {
+  createSportMock.mockResolvedValue(err(Errors.conflict('Ya existe un deporte con ese nombre.')));
+
+  const payload: CreateSportInput = { name: 'Tenis' };
+  const { c, json } = createContext(payload);
+
+  const response = await createSportHandler(c as never);
+
+  expect(createSportMock).toHaveBeenCalledWith(payload);
+  expect(json.mock.calls[0]?.[0]).toEqual({
+    type: 'https://api.myapp.dev/problems/conflict',
+    title: 'Conflict',
+    status: 409,
+    detail: 'Ya existe un deporte con ese nombre.',
+  });
+  expect(json.mock.calls[0]?.[1]).toBe(409);
+  expect(response).toBeDefined();
+});