Add IMAP support for email fetching and marking as seen

Introduced IMAP functionality to enable fetching emails from an
IMAP server and marking messages as seen. Updated the
`EmailAccountDto` class with IMAP-related properties
(`ImapServer`, `ImapPort`, `ImapUseSsl`) for configuration.

Added `ReceivedEmailContext` to represent received emails and
created the `IImapEmailService` interface with methods
`FetchEmailsAsync` and `MarkAsSeenAsync`. Implemented the
`LimilabsImapEmailService` class using Limilabs Mail.dll for
IMAP operations, including connection handling, email fetching,
and marking messages as seen.

Added `FetchEmailsQuery` and `MarkEmailAsSeenCommand` with
handlers to encapsulate IMAP logic. Updated `EmailsController`
with new endpoints for fetching emails and marking messages as
seen. Registered `IImapEmailService` in `DependencyInjection`.

Included exception handling and logging for robust error
management during IMAP operations.

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IImapEmailService.cs

@@ -0,0 +1,33 @@
+using DigitalData.MessagingService.Abstraction;
+
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// Service interface for reading emails via IMAP.
+/// </summary>
+public interface IImapEmailService
+{
+    /// <summary>
+    /// Fetches emails from the specified mailbox folder.
+    /// </summary>
+    /// <param name="account">Account whose IMAP settings will be used.</param>
+    /// <param name="folder">Mailbox folder name (e.g. "INBOX"). Defaults to INBOX.</param>
+    /// <param name="unseenOnly">When <see langword="true"/> returns only unread messages.</param>
+    /// <param name="maxCount">Maximum number of messages to retrieve (most-recent first). 0 = unlimited.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<IEnumerable<ReceivedEmailContext>> FetchEmailsAsync(
+        EmailAccountDto account,
+        string folder = "INBOX",
+        bool unseenOnly = false,
+        int maxCount = 50,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Marks a message as seen (read) on the server.
+    /// </summary>
+    Task MarkAsSeenAsync(
+        EmailAccountDto account,
+        long uid,
+        string folder = "INBOX",
+        CancellationToken cancellationToken = default);
+}