feat: migrate AuthController to FastEndpoints Endpoints (Phase 1)

IMPLEMENTATION:
- Create 4 FastEndpoints Endpoint classes:
  - LoginEndpoint: POST /api/auth/login
  - RefreshTokenEndpoint: POST /api/auth/refresh
  - ChangePasswordEndpoint: POST /api/auth/change-password
  - ResetPasswordEndpoint: POST /api/auth/reset-password

- Backup AuthController.cs (no longer active)
- Add FastEndpoints.Endpoint<TRequest, TResponse> pattern
- Implement proper DI with AuthService injection
- Use Policies("Bearer") for authorization
- Proper error handling with ThrowError()

ARCHITECTURE:
- Start of Phase 1: Core Auth APIs
- Endpoints follow FastEndpoints conventions
- DTOs: LoginRequest, RefreshTokenRequest, ChangePasswordRequest, ResetPasswordRequest, TokenPairResponse, MessageResponse
- AllowAnonymous for login/refresh/reset
- Bearer policy for change-password

VERIFICATION:
✅ dotnet build: 0 errors, 0 warnings
✅ dotnet test: 26/26 passed
✅ FastEndpoints auto-discovery working (no endpoint errors)
✅ JWT validation passes

Next Phase: BlogController and remaining APIs

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

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