feat(domain): add IncomingProtocol enum (None, Imap, Pop3, ImapOAuth2, Pop3OAuth2)

src/core/DigitalData.MessagingService.Domain/Enums/IncomingProtocol.cs

@@ -0,0 +1,34 @@
+namespace DigitalData.MessagingService.Domain.Enums;
+
+/// <summary>
+/// Specifies the protocol used to receive (sync) incoming emails for an account.
+/// </summary>
+public enum IncomingProtocol
+{
+    /// <summary>
+    /// No incoming protocol configured — account is send-only.
+    /// </summary>
+    None = 0,
+
+    /// <summary>
+    /// Sync emails via IMAP using username/password authentication.
+    /// </summary>
+    Imap = 1,
+
+    /// <summary>
+    /// Sync emails via POP3 using username/password authentication.
+    /// </summary>
+    Pop3 = 2,
+
+    /// <summary>
+    /// Sync emails via IMAP using OAuth2 (XOAUTH2) authentication.
+    /// Requires <c>UseOAuth2 = true</c> and valid OAuth2 credentials.
+    /// </summary>
+    ImapOAuth2 = 3,
+
+    /// <summary>
+    /// Sync emails via POP3 using OAuth2 (XOAUTH2) authentication.
+    /// Requires <c>UseOAuth2 = true</c> and valid OAuth2 credentials.
+    /// </summary>
+    Pop3OAuth2 = 4,
+}