feat(domain): add common base classes and interfaces

- Add BaseEntity with audit fields (CreatedDate, CreatedBy, ModifiedDate, ModifiedBy)
- Add IAggregateRoot marker interface for DDD aggregate roots
- Add ValueObject base class with equality comparison by value

These classes provide the foundation for all domain entities and value objects.

src/DigitalData.EmailProfiler.Domain/Common/BaseEntity.cs

@@ -0,0 +1,9 @@
+namespace DigitalData.EmailProfiler.Domain.Common;
+
+public abstract class BaseEntity
+{
+    public DateTime? CreatedDate { get; set; }
+    public string? CreatedBy { get; set; }
+    public DateTime? ModifiedDate { get; set; }
+    public string? ModifiedBy { get; set; }
+}

src/DigitalData.EmailProfiler.Domain/Common/IAggregateRoot.cs

@@ -0,0 +1,8 @@
+namespace DigitalData.EmailProfiler.Domain.Common;
+
+/// <summary>
+/// Marker interface for aggregate roots in DDD
+/// </summary>
+public interface IAggregateRoot
+{
+}

src/DigitalData.EmailProfiler.Domain/Common/ValueObject.cs

@@ -0,0 +1,43 @@
+namespace DigitalData.EmailProfiler.Domain.Common;
+
+/// <summary>
+/// Base class for value objects that implement equality by value
+/// </summary>
+public abstract class ValueObject
+{
+    protected abstract IEnumerable<object> GetEqualityComponents();
+
+    public override bool Equals(object? obj)
+    {
+        if (obj == null || obj.GetType() != GetType())
+        {
+            return false;
+        }
+
+        var other = (ValueObject)obj;
+        return GetEqualityComponents().SequenceEqual(other.GetEqualityComponents());
+    }
+
+    public override int GetHashCode()
+    {
+        return GetEqualityComponents()
+            .Select(x => x?.GetHashCode() ?? 0)
+            .Aggregate((x, y) => x ^ y);
+    }
+
+    public static bool operator ==(ValueObject? left, ValueObject? right)
+    {
+        if (left is null && right is null)
+            return true;
+
+        if (left is null || right is null)
+            return false;
+
+        return left.Equals(right);
+    }
+
+    public static bool operator !=(ValueObject? left, ValueObject? right)
+    {
+        return !(left == right);
+    }
+}