feat: implement immutable Lead entity and repository interface

- Add Lead record with init properties for immutability
- Use DateTimeOffset for UTC-safe timestamps
- Implement domain validation methods (email, GUID, full name)
- Add ILeadRepository with async methods and idempotency support
- Follow Clean Architecture and SOLID principles

Author: sl-cloud

src/LeadProcessor.Domain/Entities/Lead.cs

@@ -0,0 +1,118 @@
+namespace LeadProcessor.Domain.Entities;
+
+/// <summary>
+/// Represents a lead entity captured from various sources.
+/// </summary>
+public record Lead
+{
+    /// <summary>
+    /// Gets the unique identifier for the lead.
+    /// </summary>
+    public int Id { get; init; }
+
+    /// <summary>
+    /// Gets the tenant identifier for multi-tenancy support.
+    /// </summary>
+    public required string TenantId { get; init; }
+
+    /// <summary>
+    /// Gets the correlation identifier for idempotency and message tracking.
+    /// </summary>
+    public required string CorrelationId { get; init; }
+
+    /// <summary>
+    /// Gets the email address of the lead.
+    /// </summary>
+    public required string Email { get; init; }
+
+    /// <summary>
+    /// Gets the first name of the lead.
+    /// </summary>
+    public string? FirstName { get; init; }
+
+    /// <summary>
+    /// Gets the last name of the lead.
+    /// </summary>
+    public string? LastName { get; init; }
+
+    /// <summary>
+    /// Gets the phone number of the lead.
+    /// </summary>
+    public string? Phone { get; init; }
+
+    /// <summary>
+    /// Gets the company name of the lead.
+    /// </summary>
+    public string? Company { get; init; }
+
+    /// <summary>
+    /// Gets the source from which the lead originated (e.g., website, mobile app, referral).
+    /// </summary>
+    public required string Source { get; init; }
+
+    /// <summary>
+    /// Gets the metadata as a JSON string containing additional information about the lead.
+    /// </summary>
+    public string? Metadata { get; init; }
+
+    /// <summary>
+    /// Gets the UTC date and time when the lead was created.
+    /// </summary>
+    public DateTimeOffset CreatedAt { get; init; }
+
+    /// <summary>
+    /// Gets the UTC date and time when the lead was last updated.
+    /// </summary>
+    public DateTimeOffset UpdatedAt { get; init; }
+
+    /// <summary>
+    /// Validates that the email format is correct.
+    /// </summary>
+    /// <returns>
+    /// True if the email is in a valid format according to RFC 5322, otherwise false.
+    /// Returns false if email is null, empty, or whitespace.
+    /// </returns>
+    public bool HasValidEmail()
+    {
+        if (string.IsNullOrWhiteSpace(Email))
+            return false;
+
+        try
+        {
+            var addr = new System.Net.Mail.MailAddress(Email);
+            return addr.Address == Email;
+        }
+        catch (FormatException)
+        {
+            return false;
+        }
+    }
+
+    /// <summary>
+    /// Validates that the correlation ID is in GUID format.
+    /// </summary>
+    /// <returns>True if the correlation ID can be parsed as a GUID, otherwise false.</returns>
+    public bool IsCorrelationIdGuid()
+    {
+        return Guid.TryParse(CorrelationId, out _);
+    }
+
+    /// <summary>
+    /// Gets the full name of the lead by combining first and last names.
+    /// </summary>
+    /// <returns>The full name, or null if both names are empty.</returns>
+    public string? GetFullName()
+    {
+        var hasFirst = !string.IsNullOrWhiteSpace(FirstName);
+        var hasLast = !string.IsNullOrWhiteSpace(LastName);
+
+        return (hasFirst, hasLast) switch
+        {
+            (true, true) => $"{FirstName} {LastName}",
+            (true, false) => FirstName,
+            (false, true) => LastName,
+            _ => null
+        };
+    }
+}
+

src/LeadProcessor.Domain/Repositories/ILeadRepository.cs

@@ -0,0 +1,35 @@
+using LeadProcessor.Domain.Entities;
+
+namespace LeadProcessor.Domain.Repositories;
+
+/// <summary>
+/// Repository interface for managing lead persistence operations.
+/// </summary>
+public interface ILeadRepository
+{
+    /// <summary>
+    /// Saves a lead to the data store asynchronously.
+    /// </summary>
+    /// <param name="lead">The lead entity to save.</param>
+    /// <param name="cancellationToken">Cancellation token to cancel the operation.</param>
+    /// <returns>The saved lead entity with updated fields (e.g., Id, timestamps).</returns>
+    Task<Lead> SaveLeadAsync(Lead lead, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Checks if a lead with the specified correlation ID already exists.
+    /// This method supports idempotency by allowing duplicate message detection.
+    /// </summary>
+    /// <param name="correlationId">The correlation ID to check for.</param>
+    /// <param name="cancellationToken">Cancellation token to cancel the operation.</param>
+    /// <returns>True if a lead with the correlation ID exists, otherwise false.</returns>
+    Task<bool> ExistsByCorrelationIdAsync(string correlationId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Retrieves a lead by its correlation ID asynchronously.
+    /// </summary>
+    /// <param name="correlationId">The correlation ID to search for.</param>
+    /// <param name="cancellationToken">Cancellation token to cancel the operation.</param>
+    /// <returns>The lead entity if found, otherwise null.</returns>
+    Task<Lead?> GetByCorrelationIdAsync(string correlationId, CancellationToken cancellationToken = default);
+}
+