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/EmailReceiving/Queries/FetchEmailsQuery.cs

@@ -0,0 +1,56 @@
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Application.EmailAccount.Queries;
+using DigitalData.MessagingService.Abstraction;
+using DigitalData.MessagingService.Domain.Exceptions;
+using MediatR;
+
+namespace DigitalData.MessagingService.Application.EmailReceiving.Queries;
+
+/// <summary>
+/// Query to fetch emails from an IMAP mailbox.
+/// </summary>
+public record FetchEmailsQuery : IRequest<IEnumerable<ReceivedEmailContext>>
+{
+    /// <summary>
+    /// Identifies the email account to use.
+    /// </summary>
+    public required GetSenderQuery Account { get; init; }
+
+    /// <summary>
+    /// Mailbox folder to read from (default: "INBOX").
+    /// </summary>
+    public string Folder { get; init; } = "INBOX";
+
+    /// <summary>
+    /// When <see langword="true"/> returns only unread messages.
+    /// </summary>
+    public bool UnseenOnly { get; init; } = false;
+
+    /// <summary>
+    /// Maximum number of messages to retrieve (most-recent first). 0 = unlimited.
+    /// </summary>
+    public int MaxCount { get; init; } = 50;
+}
+
+public class FetchEmailsQueryHandler(
+    ISender Sender,
+    IImapEmailService ImapService) : IRequestHandler<FetchEmailsQuery, IEnumerable<ReceivedEmailContext>>
+{
+    public async Task<IEnumerable<ReceivedEmailContext>> Handle(FetchEmailsQuery request, CancellationToken cancellationToken)
+    {
+        var account = await Sender.Send(request.Account, cancellationToken)
+            ?? throw new NotFoundException(
+                $"No email account found for the given criteria (Id: {request.Account.Id}, Username: {request.Account.Username}).");
+
+        if (string.IsNullOrWhiteSpace(account.ImapServer))
+            throw new InvalidOperationException(
+                $"IMAP is not configured for account '{account.Username}' (Id: {account.Id}). Set ImapServer in EmailAccounts configuration.");
+
+        return await ImapService.FetchEmailsAsync(
+            account,
+            request.Folder,
+            request.UnseenOnly,
+            request.MaxCount,
+            cancellationToken);
+    }
+}