추가: 초기 관리자 계정 및 블로그 포스트 5개

- 초기 관리자: admin / admin123
- 블로그 포스트 5개 자동 생성
- AdminUserRepository 구현
- CreateBlogPostDto 추가

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

TaxBaik.Infrastructure/Repositories/AdminUserRepository.cs

@@ -0,0 +1,34 @@
+using Dapper;
+using TaxBaik.Domain.Entities;
+using TaxBaik.Domain.Interfaces;
+
+namespace TaxBaik.Infrastructure.Repositories;
+
+public class AdminUserRepository : BaseRepository, IAdminUserRepository
+{
+    public AdminUserRepository(IDbConnectionFactory connectionFactory) : base(connectionFactory) { }
+
+    public async Task<AdminUser?> GetByUsernameAsync(string username)
+    {
+        using var conn = _connectionFactory.CreateConnection();
+        return await conn.QueryFirstOrDefaultAsync<AdminUser>(
+            "SELECT * FROM admin_users WHERE username = @username",
+            new { username });
+    }
+
+    public async Task<AdminUser?> GetByIdAsync(int id)
+    {
+        using var conn = _connectionFactory.CreateConnection();
+        return await conn.QueryFirstOrDefaultAsync<AdminUser>(
+            "SELECT * FROM admin_users WHERE id = @id",
+            new { id });
+    }
+
+    public async Task CreateAsync(AdminUser user)
+    {
+        using var conn = _connectionFactory.CreateConnection();
+        await conn.ExecuteAsync(
+            "INSERT INTO admin_users (username, password_hash, created_at) VALUES (@username, @passwordHash, NOW())",
+            new { username = user.Username, passwordHash = user.PasswordHash });
+    }
+}