Add comprehensive test suite infrastructure

- Create backend xUnit test project with Moq and FluentAssertions
- Add test utilities: TestDataFactory, MockHttpContext, TestUserClaims
- Create AuthControllerTests with comprehensive auth scenarios
- Install Jest and React Testing Library for frontend
- Configure jest.config.ts and jest.setup.ts with Next.js support
- Add test scripts to package.json

openspec/changes/add-tests/specs/backend-unit-tests/spec.md

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+## ADDED Requirements
+
+### Requirement: Backend test project exists
+The system SHALL have a dedicated .NET test project named `backend.Tests` that references the main `backend` project.
+
+#### Scenario: Test project references main project
+- **WHEN** the test project is built
+- **THEN** it SHALL have a project reference to the main backend API project
+
+### Requirement: Controllers can be unit tested
+The system SHALL provide the ability to unit test API Controllers with mocked dependencies.
+
+#### Scenario: Controller with mocked service
+- **WHEN** a controller action is invoked with a mocked service
+- **THEN** the test SHALL verify the correct HTTP response is returned
+
+#### Scenario: Controller input validation
+- **WHEN** invalid input is passed to a controller action
+- **THEN** the test SHALL verify validation errors are returned
+
+### Requirement: Services can be unit tested
+The system SHALL provide the ability to unit test business logic in Services with mocked dependencies.
+
+#### Scenario: Service with mocked repository
+- **WHEN** a service method is called with a mocked data repository
+- **THEN** the test SHALL verify the expected business logic is executed
+
+### Requirement: Test utilities are available
+The system SHALL provide common test utilities for setup, teardown, and assertions.
+
+#### Scenario: Mock data factories
+- **WHEN** tests require test data
+- **THEN** factory classes SHALL generate consistent mock entities
+
+#### Scenario: HTTP context mocking
+- **WHEN** tests need to simulate HTTP requests
+- **THEN** utilities SHALL provide mocked HttpContext and User claims