Add BaseModel and AppDbContext with auto timestamps

- BaseModel in Domain.Common with Id, Created, Updated, Deleted fields
- AppDbContext in Infrastructure.Persistence overrides SaveChanges/Async
  to automatically set Created/Updated via ChangeTracker
- Added EF Core 8 and Pomelo.EntityFrameworkCore.MySql 8 packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/ClaudeTest.Domain/Common/BaseModel.cs

@@ -0,0 +1,9 @@
+namespace ClaudeTest.Domain.Common;
+
+public abstract class BaseModel
+{
+    public int Id { get; set; }
+    public DateTime Created { get; set; }
+    public DateTime Updated { get; set; }
+    public DateTime? Deleted { get; set; }
+}

backend/ClaudeTest.Infrastructure/ClaudeTest.Infrastructure.csproj

@@ -5,6 +5,11 @@
     <ProjectReference Include="..\ClaudeTest.Domain\ClaudeTest.Domain.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="8.*" />
+    <PackageReference Include="Pomelo.EntityFrameworkCore.MySql" Version="8.*" />
+  </ItemGroup>
+
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>

backend/ClaudeTest.Infrastructure/Persistence/AppDbContext.cs

@@ -0,0 +1,42 @@
+using ClaudeTest.Domain.Common;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeTest.Infrastructure.Persistence;
+
+public class AppDbContext : DbContext
+{
+    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
+    {
+    }
+
+    public override int SaveChanges()
+    {
+        SetTimestamps();
+        return base.SaveChanges();
+    }
+
+    public override Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
+    {
+        SetTimestamps();
+        return base.SaveChangesAsync(cancellationToken);
+    }
+
+    private void SetTimestamps()
+    {
+        var entries = ChangeTracker.Entries<BaseModel>();
+
+        foreach (var entry in entries)
+        {
+            switch (entry.State)
+            {
+                case EntityState.Added:
+                    entry.Entity.Created = DateTime.UtcNow;
+                    entry.Entity.Updated = DateTime.UtcNow;
+                    break;
+                case EntityState.Modified:
+                    entry.Entity.Updated = DateTime.UtcNow;
+                    break;
+            }
+        }
+    }
+}