refactor: Web과 Admin 통합 - 단일 포트 5001로 운영

분리의 단점을 제거하고 단일 앱으로 통합:

구조 변경:
- TaxBaik.Admin → TaxBaik.Web/Components/Admin/
- Admin Services → TaxBaik.Web/Services/
- 포트: 5001 (기존 5002 제거)

경로:
- 홈페이지: http://localhost:5001/taxbaik
- 관리자: http://localhost:5001/taxbaik/admin

기술:
- Razor Pages (Web) + Blazor Server (Admin) 통합
- 단일 Program.cs로 양쪽 모두 지원
- JWT 인증 유지
- MudBlazor UI 유지

장점:
- 개발 복잡도 감소 (터미널 1개)
- 배포 단순화 (앱 1개)
- DB 마이그레이션 1회 실행

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

TaxBaik.Web/Services/AuthService.cs

@@ -0,0 +1,96 @@
+using System.IdentityModel.Tokens.Jwt;
+using System.Security.Claims;
+using System.Text;
+using BCrypt.Net;
+using Microsoft.IdentityModel.Tokens;
+using TaxBaik.Domain.Entities;
+using TaxBaik.Domain.Interfaces;
+
+namespace TaxBaik.Web.Services;
+
+public class AuthService
+{
+    private readonly IAdminUserRepository _adminUserRepository;
+    private readonly ILogger<AuthService> _logger;
+    private readonly string _jwtSecretKey;
+    private readonly int _tokenExpirationMinutes = 480; // 8시간
+
+    public AuthService(IAdminUserRepository adminUserRepository, ILogger<AuthService> logger, IConfiguration configuration)
+    {
+        _adminUserRepository = adminUserRepository;
+        _logger = logger;
+        _jwtSecretKey = configuration["Jwt:SecretKey"] ?? throw new InvalidOperationException("Missing 'Jwt:SecretKey' configuration.");
+    }
+
+    public async Task<string?> AuthenticateAndGenerateTokenAsync(string username, string password)
+    {
+        if (string.IsNullOrWhiteSpace(username) || string.IsNullOrWhiteSpace(password))
+        {
+            return null;
+        }
+
+        var user = await _adminUserRepository.GetByUsernameAsync(username);
+        if (user == null)
+        {
+            _logger.LogWarning("로그인 시도: 존재하지 않는 사용자 '{Username}'", username);
+            return null;
+        }
+
+        if (!BCrypt.Net.BCrypt.Verify(password, user.PasswordHash))
+        {
+            _logger.LogWarning("로그인 시도: 잘못된 비밀번호 '{Username}'", username);
+            return null;
+        }
+
+        _logger.LogInformation("로그인 성공: {Username}", username);
+        return GenerateJwtToken(user);
+    }
+
+    private string GenerateJwtToken(AdminUser user)
+    {
+        var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_jwtSecretKey));
+        var creds = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);
+
+        var claims = new[]
+        {
+            new Claim(ClaimTypes.NameIdentifier, user.Id.ToString()),
+            new Claim(ClaimTypes.Name, user.Username),
+            new Claim("username", user.Username)
+        };
+
+        var token = new JwtSecurityToken(
+            issuer: "taxbaik-admin",
+            audience: "taxbaik-admin-client",
+            claims: claims,
+            expires: DateTime.UtcNow.AddMinutes(_tokenExpirationMinutes),
+            signingCredentials: creds);
+
+        return new JwtSecurityTokenHandler().WriteToken(token);
+    }
+
+    public ClaimsPrincipal? ValidateToken(string token)
+    {
+        try
+        {
+            var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_jwtSecretKey));
+            var handler = new JwtSecurityTokenHandler();
+            var principal = handler.ValidateToken(token, new TokenValidationParameters
+            {
+                ValidateIssuerSigningKey = true,
+                IssuerSigningKey = key,
+                ValidateIssuer = true,
+                ValidIssuer = "taxbaik-admin",
+                ValidateAudience = true,
+                ValidAudience = "taxbaik-admin-client",
+                ValidateLifetime = true,
+                ClockSkew = TimeSpan.Zero
+            }, out SecurityToken validatedToken);
+
+            return principal;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+}