Refactor namespaces and remove Abstraction project

Replaced `DigitalData.MessagingService.Abstraction` with
`DigitalData.MessagingService.Application.Common.Dto` and
`DigitalData.MessagingService.Application.Common.Dto.MailSearch`
to improve modularity and organization.

Removed the `Abstraction` project and updated all references
to use the `Application` project. Updated namespaces, `using`
directives, and dependencies across the codebase.

Refactored interfaces, commands, queries, validators, and
services to use the new DTOs. Updated RabbitMQ integration,
AutoMapper profiles, background services, and tests to align
with the new structure.

Performed general cleanup by removing redundant `using`
directives and obsolete references.

src/core/DigitalData.MessagingService.Application/Common/Dto/EmailAccountDto.cs

@@ -0,0 +1,52 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+/// <summary>
+/// DTO for a single email account configuration.
+/// </summary>
+public class EmailAccountDto
+{
+    /// <summary>
+    /// Logical name to identify this account (e.g. "default", "support").
+    /// </summary>
+    public int Id { get; set; }
+
+#if NET
+    public required string Username { get; set; }
+#else
+    public string Username { get; set; } = null!;
+#endif
+
+#if NET
+    public required string Password { get; set; }
+#else
+    public string Password { get; set; } = null!;
+#endif
+
+#if NET
+    public required string SmtpServer { get; set; }
+#else
+    public string SmtpServer { get; set; } = null!;
+#endif
+
+    public int SmtpPort { get; set; }
+
+    public bool SmtpUseSsl { get; set; }
+
+    public bool UseOAuth2 { get; set; }
+
+    /// <summary>
+    /// IMAP server hostname (e.g. "imap.example.com").
+    /// Leave empty when this account is send-only.
+    /// </summary>
+    public string? ImapServer { get; set; }
+
+    /// <summary>
+    /// IMAP server port (993 for SSL, 143 for plain/STARTTLS).
+    /// </summary>
+    public int ImapPort { get; set; } = 993;
+
+    /// <summary>
+    /// Use SSL/TLS when connecting to the IMAP server.
+    /// </summary>
+    public bool ImapUseSsl { get; set; } = true;
+}

src/core/DigitalData.MessagingService.Application/Common/Dto/EmailAttachmentDto.cs

@@ -0,0 +1,45 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+/// <summary>
+/// Represents a single email attachment.
+/// </summary>
+public sealed class EmailAttachmentDto
+{
+    /// <summary>
+    /// Display name of the attachment (e.g. "invoice.pdf").
+    /// </summary>
+#if NETFRAMEWORK
+    public string FileName { get; set; } = null!;
+#else
+    public required string FileName { get; init; }
+#endif
+
+    /// <summary>
+    /// Raw content of the attachment.
+    /// </summary>
+#if NETFRAMEWORK
+    public byte[] Content { get; set; } = null!;
+#else
+    public required byte[] Content { get; init; }
+#endif
+
+    /// <summary>
+    /// MIME content-type (e.g. "application/pdf", "image/png").
+    /// Defaults to "application/octet-stream" when not specified.
+    /// </summary>
+    public string ContentType { get; set; } = "application/octet-stream";
+
+    /// <summary>
+    /// When <see langword="true"/> the attachment is embedded inline and displayed
+    /// directly inside the email body via a CID reference (e.g. &lt;img src="cid:logo"&gt;).
+    /// When <see langword="false"/> (default) it appears as a regular downloadable attachment.
+    /// </summary>
+    public bool IsInline { get; set; } = false;
+
+    /// <summary>
+    /// Content-ID used when <see cref="IsInline"/> is <see langword="true"/>.
+    /// Reference it in HTML body as <c>cid:{ContentId}</c>.
+    /// Auto-generated from <see cref="FileName"/> when left empty.
+    /// </summary>
+    public string? ContentId { get; set; }
+}

src/core/DigitalData.MessagingService.Application/Common/Dto/EmailContext.cs

@@ -0,0 +1,55 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+public record EmailContext
+{
+#if NETFRAMEWORK
+    public EmailAccountDto Sender { get; set; } = null!;
+#else
+    public required EmailAccountDto Sender { get; init; }
+#endif
+
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+
+#if NETFRAMEWORK
+    public IEnumerable<string> Recipients { get; set; } = null!;
+#else
+    public required IEnumerable<string> Recipients { get; init; }
+#endif
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+
+#if NETFRAMEWORK
+    public string Subject { get; set; } = null!;
+#else
+    public required string Subject { get; init; }
+#endif
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+#if NETFRAMEWORK
+    public string Body { get; set; } = null!;
+#else
+    public required string Body { get; init; }
+#endif
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+#if NETFRAMEWORK
+    public bool IsHtml { get; set; } = true;
+#else
+    public bool IsHtml { get; init; } = true;
+#endif
+
+    /// <summary>
+    /// Optional list of attachments to include with the email.
+    /// Each entry may carry its content as a byte array (<see cref="EmailAttachmentDto.Content"/>)
+    /// or reference a file on disk via <see cref="EmailAttachmentDto.FilePath"/>.
+    /// </summary>
+    public IEnumerable<EmailAttachmentDto> Attachments { get; set; } = [];
+}

src/core/DigitalData.MessagingService.Application/Common/Dto/MailSearch/DateFilter.cs

@@ -1,5 +1,5 @@
 #if NET
-namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
+namespace DigitalData.MessagingService.Application.Common.Dto.MailSearch;
 
 /// <summary>
 /// Constrains the search to messages within an arrival-date range.

src/core/DigitalData.MessagingService.Application/Common/Dto/MailSearch/MailSearchFilter.cs

@@ -1,5 +1,5 @@
 #if NET
-namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
+namespace DigitalData.MessagingService.Application.Common.Dto.MailSearch;
 
 /// <summary>
 /// Describes all criteria and options used when searching an IMAP mailbox.

src/core/DigitalData.MessagingService.Application/Common/Dto/MailSearch/MailSortOrder.cs

@@ -1,4 +1,4 @@
-namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
+namespace DigitalData.MessagingService.Application.Common.Dto.MailSearch;
 
 /// <summary>
 /// Controls the order in which fetched messages are returned.

src/core/DigitalData.MessagingService.Application/Common/Dto/MailSearch/UidFilter.cs

@@ -1,5 +1,5 @@
 #if NET
-namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
+namespace DigitalData.MessagingService.Application.Common.Dto.MailSearch;
 
 /// <summary>
 /// Constrains the search to a specific UID or a UID range.

src/core/DigitalData.MessagingService.Application/Common/Dto/ReceivedEmailDto.cs

@@ -0,0 +1,97 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+/// <summary>
+/// Represents an email message received via IMAP.
+/// </summary>
+public sealed record ReceivedEmailDto
+{
+    /// <summary>
+    /// Unique identifier of the message on the IMAP server (UID).
+    /// </summary>
+#if NET
+    public long Uid { get; init; }
+#else
+    public long Uid { get; set; }
+#endif
+
+    /// <summary>
+    /// Sender address (From header).
+    /// </summary>
+#if NET
+    public string From { get; init; } = string.Empty;
+#else
+    public string From { get; set; } = string.Empty;
+#endif
+
+    /// <summary>
+    /// Recipient addresses (To header).
+    /// </summary>
+#if NET
+    public IEnumerable<string> To { get; init; } = [];
+#else
+    public IEnumerable<string> To { get; set; } = [];
+#endif
+
+    /// <summary>
+    /// CC addresses.
+    /// </summary>
+#if NET
+    public IEnumerable<string> Cc { get; init; } = [];
+#else
+    public IEnumerable<string> Cc { get; set; } = [];
+#endif
+
+    /// <summary>
+    /// Email subject.
+    /// </summary>
+#if NET
+    public string Subject { get; init; } = string.Empty;
+#else
+    public string Subject { get; set; } = string.Empty;
+#endif
+
+    /// <summary>
+    /// Plain-text body (may be empty when only HTML is present).
+    /// </summary>
+#if NET
+    public string TextBody { get; init; } = string.Empty;
+#else
+    public string TextBody { get; set; } = string.Empty;
+#endif
+
+    /// <summary>
+    /// HTML body (may be empty when only plain-text is present).
+    /// </summary>
+#if NET
+    public string HtmlBody { get; init; } = string.Empty;
+#else
+    public string HtmlBody { get; set; } = string.Empty;
+#endif
+
+    /// <summary>
+    /// Date/time the message was sent (Date header).
+    /// </summary>
+#if NET
+    public DateTime Date { get; init; }
+#else
+    public DateTime Date { get; set; }
+#endif
+
+    /// <summary>
+    /// Attachments included with this message.
+    /// </summary>
+#if NET
+    public IEnumerable<EmailAttachmentDto> Attachments { get; init; } = [];
+#else
+    public IEnumerable<EmailAttachmentDto> Attachments { get; set; } = [];
+#endif
+
+    /// <summary>
+    /// Whether the message has been marked as seen/read on the server.
+    /// </summary>
+#if NET
+    public bool IsSeen { get; init; }
+#else
+    public bool IsSeen { get; set; }
+#endif
+}

src/core/DigitalData.MessagingService.Application/Common/Dto/SendingEmailCreateDto.cs

@@ -0,0 +1,26 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+public record SendingEmailCreateDto
+{
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+    public string Recipient { get; set; } = null!;
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+    public string Subject { get; set; } = null!;
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+    public string Body { get; set; } = null!;
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+    public bool IsHtml { get; set; } = true;
+
+    public DateTime QueuedAt { get; set; }
+}

src/core/DigitalData.MessagingService.Application/Common/Dto/SendingEmailEvent.cs

@@ -0,0 +1,22 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+public record SendingEmailEvent
+{
+#if NETFRAMEWORK
+    public Guid Id { get; set; }
+#else
+    public required Guid Id { get; init; }
+#endif
+
+#if NETFRAMEWORK
+    public EmailContext Mail { get; set; } = null!;
+#else
+    public required EmailContext Mail { get; init; }
+#endif
+
+#if NETFRAMEWORK
+    public DateTime QueuedAt { get; set; }
+#else
+    public required DateTime QueuedAt { get; init; }
+#endif
+}

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

@@ -1,4 +1,4 @@
-using DigitalData.MessagingService.Abstraction;
+using DigitalData.MessagingService.Application.Common.Dto;
 
 namespace DigitalData.MessagingService.Application.Common.Interfaces;
 

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

@@ -1,6 +1,6 @@
 #if NET
-using DigitalData.MessagingService.Abstraction;
-using DigitalData.MessagingService.Application.Common.Models.MailSearch;
+using DigitalData.MessagingService.Application.Common.Dto;
+using DigitalData.MessagingService.Application.Common.Dto.MailSearch;
 
 namespace DigitalData.MessagingService.Application.Common.Interfaces;
 

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

@@ -0,0 +1,13 @@
+using DigitalData.MessagingService.Application.Common.Dto;
+
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// Email queue interface for outgoing emails.
+/// </summary>
+public interface ISendingEmailPublisher
+{
+    Task EnqueueAsync(SendingEmailEvent sendingEmailEvent, CancellationToken cancellationToken = default);
+
+    Task<int> GetQueueDepthAsync(CancellationToken cancellationToken = default);
+}

src/core/DigitalData.MessagingService.Application/Common/Mappings/EmailMappingProfile.cs

@@ -1,7 +1,7 @@
 #if NET
 using AutoMapper;
 using DigitalData.MessagingService.Application.EmailSending.Commands;
-using DigitalData.MessagingService.Abstraction;
+using DigitalData.MessagingService.Application.Common.Dto;
 
 namespace DigitalData.MessagingService.Application.Common.Mappings;
 

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

@@ -1,5 +1,5 @@
 #if NET
-using DigitalData.MessagingService.Abstraction;
+using DigitalData.MessagingService.Application.Common.Dto;
 
 namespace DigitalData.MessagingService.Application.Common.Options;