refactor(application): replace ISender dispatch with direct IRepository usage in email handlers

- FetchEmailsQuery, MarkEmailAsSeenCommand, PublishEmailCommand: resolve EmailAccount
  via IRepository<EmailAccount> instead of dispatching GetEmailAccountQuery through ISender
- Swap InvalidOperationException for BadRequestException when IMAP server is not configured
- Add ILogger to handlers; warn when multiple accounts match the lookup criteria
- EmailAccountsOptions.Accounts now typed as IEnumerable<EmailAccountModificationDto>
  instead of IEnumerable<EmailAccount> to decouple config from domain entity
- EmailMappingProfile: add EmailAccount <-> EmailAccountDto/EmailAccountModificationDto maps

src/core/DigitalData.MessagingService.Application/Common/Options/EmailAccountsOptions.cs

@@ -1,4 +1,7 @@
 #if NET
+using DigitalData.MessagingService.Application.Common.Dto;
+using DigitalData.MessagingService.Application.Common.Dto.EmailAccounts;
+using DigitalData.MessagingService.Application.EmailAccounts.Commands;
 using DigitalData.MessagingService.Domain.Entities;
 
 namespace DigitalData.MessagingService.Application.Common.Options;
@@ -14,7 +17,7 @@ public class EmailAccountsOptions
     /// <summary>
     /// The list of configured email accounts.
     /// </summary>
-    public required IEnumerable<EmailAccount> Accounts { get; init; } = [];
+    public required IEnumerable<EmailAccountModificationDto> Accounts { get; init; } = [];
 
     /// <summary>
     /// How often the IMAP sync worker polls for new emails, in seconds.