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- Backup AuthController.cs (no longer active)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>

src/TaxBaik.Web/Endpoints/Auth/LoginEndpoint.cs

@@ -0,0 +1,56 @@
+using FastEndpoints;
+using TaxBaik.Web.Services;
+
+namespace TaxBaik.Web.Endpoints.Auth;
+
+public class LoginRequest
+{
+    public string Username { get; set; } = string.Empty;
+    public string Password { get; set; } = string.Empty;
+}
+
+public class TokenPairResponse
+{
+    public string Token { get; set; } = string.Empty;
+    public string AccessToken { get; set; } = string.Empty;
+    public string RefreshToken { get; set; } = string.Empty;
+    public int ExpiresIn { get; set; }
+}
+
+public class LoginEndpoint : Endpoint<LoginRequest, TokenPairResponse>
+{
+    private readonly AuthService _authService;
+
+    public LoginEndpoint(AuthService authService)
+    {
+        _authService = authService;
+    }
+
+    public override void Configure()
+    {
+        Post("/api/auth/login");
+        AllowAnonymous();
+    }
+
+    public override async Task HandleAsync(LoginRequest request, CancellationToken ct)
+    {
+        if (string.IsNullOrWhiteSpace(request.Username) || string.IsNullOrWhiteSpace(request.Password))
+        {
+            ThrowError("로그인 정보가 필요합니다.");
+        }
+
+        var tokenPair = await _authService.AuthenticateAndGenerateTokenPairAsync(request.Username, request.Password);
+        if (tokenPair == null)
+        {
+            ThrowError("아이디 또는 비밀번호가 올바르지 않습니다.");
+        }
+
+        await SendAsync(new TokenPairResponse
+        {
+            Token = tokenPair!.AccessToken,
+            AccessToken = tokenPair.AccessToken,
+            RefreshToken = tokenPair.RefreshToken,
+            ExpiresIn = tokenPair.ExpiresIn
+        }, 200, cancellation: ct);
+    }
+}