feat(domain): add value objects for email domain

- MessageId: Unique message identifier with SHA256 hash
  Uses same algorithm as legacy system for duplicate detection compatibility
  Hash format: SHA256({originalMessageId}|{sender}|{date}|{subject})
- EmailAddress: Email address validation and parsing with name support

Value objects ensure immutability and value-based equality.

src/DigitalData.EmailProfiler.Domain/ValueObjects/EmailAddress.cs

@@ -0,0 +1,45 @@
+using System.ComponentModel.DataAnnotations;
+using DigitalData.EmailProfiler.Domain.Common;
+using DigitalData.EmailProfiler.Domain.Exceptions;
+
+namespace DigitalData.EmailProfiler.Domain.ValueObjects;
+
+/// <summary>
+/// Value object representing an email address with validation
+/// </summary>
+public class EmailAddress : ValueObject
+{
+    public string Value { get; private set; }
+    public string Domain { get; private set; }
+    public string LocalPart { get; private set; }
+
+    private EmailAddress(string value)
+    {
+        Value = value;
+        var parts = value.Split('@');
+        LocalPart = parts[0];
+        Domain = parts[1];
+    }
+
+    public static EmailAddress Create(string email)
+    {
+        if (!IsValid(email))
+            throw new DomainException($"Invalid email address: {email}");
+
+        return new EmailAddress(email.ToLowerInvariant());
+    }
+
+    private static bool IsValid(string email)
+    {
+        return !string.IsNullOrWhiteSpace(email) &&
+               email.Contains('@') &&
+               new EmailAddressAttribute().IsValid(email);
+    }
+
+    protected override IEnumerable<object> GetEqualityComponents()
+    {
+        yield return Value;
+    }
+
+    public override string ToString() => Value;
+}

src/DigitalData.EmailProfiler.Domain/ValueObjects/MessageId.cs

@@ -0,0 +1,41 @@
+using System.Security.Cryptography;
+using System.Text;
+using DigitalData.EmailProfiler.Domain.Common;
+
+namespace DigitalData.EmailProfiler.Domain.ValueObjects;
+
+/// <summary>
+/// Value object representing a unique message identifier with hash generation
+/// Uses the same algorithm as legacy system for compatibility
+/// </summary>
+public class MessageId : ValueObject
+{
+    public string Value { get; private set; }
+    public string Hash { get; private set; }
+
+    private MessageId(string value)
+    {
+        Value = value;
+        Hash = GenerateHash(value);
+    }
+
+    public static MessageId Create(string originalMessageId, string sender, DateTime date, string subject)
+    {
+        var combined = $"{originalMessageId}|{sender}|{date:yyyyMMddHHmmss}|{subject}";
+        return new MessageId(combined);
+    }
+
+    private static string GenerateHash(string input)
+    {
+        // Same algorithm as legacy: SHA256 hash
+        using var sha256 = SHA256.Create();
+        var bytes = Encoding.UTF8.GetBytes(input);
+        var hash = sha256.ComputeHash(bytes);
+        return Convert.ToBase64String(hash);
+    }
+
+    protected override IEnumerable<object> GetEqualityComponents()
+    {
+        yield return Value;
+    }
+}