新增 ApplicationDbContext 並實現時間戳功能

新增 `ApplicationDbContext` 類別，繼承自 `DbContext`，並定義 `Departments`、`Roles` 和 `Accounts` 的 `DbSet` 屬性以管理實體。在 `OnModelCreating` 方法中應用相應的實體配置。覆寫 `SaveChanges` 和 `SaveChangesAsync` 方法，並新增 `ApplyTimestamps` 方法以自動設置實體的 `CreatedAt` 和 `UpdatedAt` 時間戳。

Configurations.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.EntityFrameworkCore;
+using Models.Entities;
+using Configurations;
+
+public class ApplicationDbContext : DbContext
+{
+    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
+        : base(options)
+    {
+    }
+
+    public DbSet<Department> Departments => Set<Department>();
+    public DbSet<Role> Roles => Set<Role>();
+    public DbSet<Account> Accounts => Set<Account>();
+
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        base.OnModelCreating(modelBuilder);
+
+        // ®M¥Î©Ò¦³ EntityTypeConfiguration
+        modelBuilder.ApplyConfiguration(new DepartmentConfiguration());
+        modelBuilder.ApplyConfiguration(new RoleConfiguration());
+        modelBuilder.ApplyConfiguration(new AccountConfiguration());
+    }
+
+    public override int SaveChanges()
+    {
+        ApplyTimestamps();
+        return base.SaveChanges();
+    }
+
+    public override Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
+    {
+        ApplyTimestamps();
+        return base.SaveChangesAsync(cancellationToken);
+    }
+
+    private void ApplyTimestamps()
+    {
+        var utcNow = DateTime.UtcNow;
+
+        foreach (var entry in ChangeTracker.Entries<BaseEntity>())
+        {
+            switch (entry.State)
+            {
+                case EntityState.Added:
+                    entry.Entity.CreatedAt = utcNow;
+                    entry.Entity.UpdatedAt = null;
+                    break;
+
+                case EntityState.Modified:
+                    // ÁקK­×§ï CreatedAt
+                    entry.Property(e => e.CreatedAt).IsModified = false;
+                    entry.Entity.UpdatedAt = utcNow;
+                    break;
+            }
+        }
+    }
+}