add xmldoc comments

web/backend/Domain/Domain.Entities/Entities/UserCredential.cs

@@ -1,11 +1,38 @@
 namespace Domain.Entities;
 
+/// <summary>
+/// Represents an issued authentication credential (refresh token) for a <see cref="UserAccount"/>.
+/// Maps to the <c>UserCredential</c> table. Credentials are deleted automatically when the owning user account is deleted.
+/// </summary>
 public class UserCredential
 {
+   /// <summary>
+   /// Primary key identifying this credential. Maps to <c>UserCredentialID</c>.
+   /// </summary>
    public Guid UserCredentialId { get; set; }
+
+   /// <summary>
+   /// Foreign key referencing the owning <see cref="UserAccount"/>. Maps to <c>UserAccountID</c>.
+   /// </summary>
    public Guid UserAccountId { get; set; }
+
+   /// <summary>
+   /// The date and time the credential was issued.
+   /// </summary>
    public DateTime CreatedAt { get; set; }
+
+   /// <summary>
+   /// The date and time after which the credential is no longer valid.
+   /// </summary>
    public DateTime Expiry { get; set; }
+
+   /// <summary>
+   /// The Argon2 hash of the credential's secret value. The plaintext secret is never persisted.
+   /// </summary>
    public string Hash { get; set; } = string.Empty;
+
+   /// <summary>
+   /// SQL Server <c>ROWVERSION</c> concurrency token used to detect concurrent updates. <c>null</c> until the row has been read from the database.
+   /// </summary>
    public byte[]? Timer { get; set; }
 }