新增實體配置以定義資料庫結構

這些變更為 `Account`、`Department` 和 `Role` 實體配置資料庫結構。每個配置類別實作了 `IEntityTypeConfiguration<T>` 介面，並在 `Configure` 方法中定義了主鍵、欄位名稱、必填性和最大長度等屬性。此外，為 `Account` 實體設置了索引和外鍵關聯，以確保資料的完整性和唯一性。

Configurations/AccountConfiguration.cs

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+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using Models.Entities;
+
+namespace Configurations
+{
+    public class AccountConfiguration : IEntityTypeConfiguration<Account>
+    {
+        public void Configure(EntityTypeBuilder<Account> builder)
+        {
+            builder.ToTable("accounts");
+
+            builder.HasKey(a => a.AccountId);
+
+            builder.Property(a => a.AccountId)
+                .HasColumnName("account_id");
+
+            builder.Property(a => a.Name)
+                .HasColumnName("name")
+                .IsRequired()
+                .HasMaxLength(100);
+
+            builder.Property(a => a.Username)
+                .HasColumnName("username")
+                .IsRequired()
+                .HasMaxLength(50);
+
+            builder.Property(a => a.Password)
+                .HasColumnName("password")
+                .IsRequired()
+                .HasMaxLength(256);
+
+            builder.Property(a => a.Email)
+                .HasColumnName("email")
+                .IsRequired()
+                .HasMaxLength(254);
+
+            builder.Property(a => a.DepartmentId)
+                .HasColumnName("department_id")
+                .IsRequired();
+
+            builder.Property(a => a.RoleId)
+                .HasColumnName("role_id")
+                .IsRequired();
+
+            builder.Property(a => a.Status)
+                .HasColumnName("status")
+                .HasConversion<string>()
+                .IsRequired()
+                .HasMaxLength(20);
+
+            builder.Property(a => a.CreatedAt)
+                .HasColumnName("created_at")
+                .IsRequired();
+
+            builder.Property(a => a.UpdatedAt)
+                .HasColumnName("updated_at");
+
+            builder.Property(a => a.ModifiedBy)
+                .HasColumnName("modified_by");
+
+            // 唯一索引
+            builder.HasIndex(a => a.Username)
+                .IsUnique()
+                .HasDatabaseName("UX_accounts_username");
+
+            builder.HasIndex(a => a.Email)
+                .IsUnique()
+                .HasDatabaseName("UX_accounts_email");
+
+            // 關聯與刪除行為
+            builder.HasOne(a => a.Department)
+                .WithMany(d => d.Accounts)
+                .HasForeignKey(a => a.DepartmentId)
+                .OnDelete(DeleteBehavior.Restrict);
+
+            builder.HasOne(a => a.Role)
+                .WithMany(r => r.Accounts)
+                .HasForeignKey(a => a.RoleId)
+                .OnDelete(DeleteBehavior.Restrict);
+
+            // 自參照外鍵：modified_by -> accounts.account_id (SET NULL)
+            builder.HasOne<Account>()
+                .WithMany()
+                .HasForeignKey(a => a.ModifiedBy)
+                .OnDelete(DeleteBehavior.SetNull);
+        }
+    }
+}

Configurations/DepartmentConfiguration.cs

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+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using Models.Entities;
+
+namespace Configurations
+{
+    public class DepartmentConfiguration : IEntityTypeConfiguration<Department>
+    {
+        public void Configure(EntityTypeBuilder<Department> builder)
+        {
+            builder.ToTable("departments");
+
+            builder.HasKey(d => d.DepartmentId);
+
+            builder.Property(d => d.DepartmentId)
+                .HasColumnName("department_id");
+
+            builder.Property(d => d.DepartmentName)
+                .HasColumnName("department_name")
+                .IsRequired()
+                .HasMaxLength(100);
+        }
+    }
+}

Configurations/RoleConfiguration.cs

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+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using Models.Entities;
+
+namespace Configurations
+{
+    public class RoleConfiguration : IEntityTypeConfiguration<Role>
+    {
+        public void Configure(EntityTypeBuilder<Role> builder)
+        {
+            builder.ToTable("roles");
+
+            builder.HasKey(r => r.RoleId);
+
+            builder.Property(r => r.RoleId)
+                .HasColumnName("role_id");
+
+            builder.Property(r => r.RoleName)
+                .HasColumnName("role_name")
+                .IsRequired()
+                .HasMaxLength(50);
+        }
+    }
+}