Add email domain entities and .NET Framework support

Introduced `EmailAccount`, `EmailAttachment`, and `ReceivedEmail`
entities with database mappings using data annotations. These
entities represent email account configurations, attachments,
and received emails, respectively.

Added conditional compilation to ensure compatibility between
.NET and .NET Framework. Updated `DigitalData.MessagingService.Domain.csproj`
to include `System.ComponentModel.DataAnnotations` for `net462`.

Defined relationships between entities, including navigation
properties and foreign key constraints.

src/core/DigitalData.MessagingService.Domain/DigitalData.MessagingService.Domain.csproj

@@ -11,4 +11,8 @@
     <PackageReference Include="MediatR" Version="12.2.0" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'net462'">
+    <Reference Include="System.ComponentModel.DataAnnotations" />
+  </ItemGroup>
+
 </Project>

src/core/DigitalData.MessagingService.Domain/Entities/EmailAccount.cs

@@ -1,52 +1,75 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
 namespace DigitalData.MessagingService.Domain.Entities;
 
 /// <summary>
 /// DTO for a single email account configuration.
 /// </summary>
+[Table("EMAIL_ACCOUNT")]
 public class EmailAccount
 {
     /// <summary>
     /// Logical name to identify this account (e.g. "default", "support").
     /// </summary>
+    [Key]
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    [Column("ID", TypeName = "int")]
     public int Id { get; set; }
 
+    [Required]
+    [MaxLength(256)]
+    [Column("USERNAME", TypeName = "nvarchar(256)")]
 #if NET
     public required string Username { get; set; }
 #else
     public string Username { get; set; } = null!;
 #endif
 
+    [Required]
+    [MaxLength(512)]
+    [Column("PASSWORD", TypeName = "nvarchar(512)")]
 #if NET
     public required string Password { get; set; }
 #else
     public string Password { get; set; } = null!;
 #endif
 
+    [Required]
+    [MaxLength(256)]
+    [Column("SMTP_SERVER", TypeName = "nvarchar(256)")]
 #if NET
     public required string SmtpServer { get; set; }
 #else
     public string SmtpServer { get; set; } = null!;
 #endif
 
+    [Column("SMTP_PORT", TypeName = "int")]
     public int SmtpPort { get; set; }
 
+    [Column("SMTP_USE_SSL", TypeName = "bit")]
     public bool SmtpUseSsl { get; set; }
 
+    [Column("USE_OAUTH2", TypeName = "bit")]
     public bool UseOAuth2 { get; set; }
 
     /// <summary>
     /// IMAP server hostname (e.g. "imap.example.com").
     /// Leave empty when this account is send-only.
     /// </summary>
+    [MaxLength(256)]
+    [Column("IMAP_SERVER", TypeName = "nvarchar(256)")]
     public string? ImapServer { get; set; }
 
     /// <summary>
     /// IMAP server port (993 for SSL, 143 for plain/STARTTLS).
     /// </summary>
+    [Column("IMAP_PORT", TypeName = "int")]
     public int ImapPort { get; set; } = 993;
 
     /// <summary>
     /// Use SSL/TLS when connecting to the IMAP server.
     /// </summary>
+    [Column("IMAP_USE_SSL", TypeName = "bit")]
     public bool ImapUseSsl { get; set; } = true;
 }

src/core/DigitalData.MessagingService.Domain/Entities/EmailAttachment.cs

@@ -0,0 +1,83 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace DigitalData.MessagingService.Domain.Entities;
+
+/// <summary>
+/// Represents a single email attachment.
+/// </summary>
+[Table("EMAIL_ATTACHMENT")]
+public sealed class EmailAttachment
+{
+    [Key]
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    [Column("ID", TypeName = "bigint")]
+#if NET
+    public long Id { get; init; }
+#else
+    public long Id { get; set; }
+#endif
+
+    /// <summary>
+    /// Display name of the attachment (e.g. "invoice.pdf").
+    /// </summary>
+    [Required]
+    [MaxLength(260)]
+    [Column("FILE_NAME", TypeName = "nvarchar(260)")]
+#if NETFRAMEWORK
+    public string FileName { get; set; } = null!;
+#else
+    public required string FileName { get; init; }
+#endif
+
+    /// <summary>
+    /// Raw content of the attachment.
+    /// </summary>
+    [Required]
+    [Column("CONTENT", TypeName = "varbinary(max)")]
+#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>
+    [Required]
+    [MaxLength(256)]
+    [Column("CONTENT_TYPE", TypeName = "nvarchar(256)")]
+    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>
+    [Column("IS_INLINE", TypeName = "bit")]
+    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>
+    [MaxLength(512)]
+    [Column("CONTENT_ID", TypeName = "nvarchar(512)")]
+    public string? ContentId { get; set; }
+
+    [ForeignKey(nameof(Email))]
+    [Column("EMAIL_ID", TypeName = "bigint")]
+#if NET
+    public long EmailId { get; init; }
+#else
+    public long EmailId { get; set; }
+#endif
+
+#if NET
+    public ReceivedEmail? Email { get; init; }
+#else
+    public ReceivedEmail? Email { get; set; }
+#endif
+}

src/core/DigitalData.MessagingService.Domain/Entities/ReceivedEmail.cs

@@ -0,0 +1,136 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace DigitalData.MessagingService.Domain.Entities;
+
+/// <summary>
+/// Represents an email message received via IMAP.
+/// </summary>
+[Table("RECEIVED_EMAIL")]
+public sealed record ReceivedEmail
+{
+    [Key]
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    [Column("ID", TypeName = "bigint")]
+#if NET
+    public long Id { get; init; }
+#else
+    public long Id { get; set; }
+#endif
+
+    /// <summary>
+    /// Unique identifier of the message on the IMAP server (UID).
+    /// </summary>
+    [Column("UID", TypeName = "bigint")]
+#if NET
+    public long Uid { get; init; }
+#else
+    public long Uid { get; set; }
+#endif
+
+    /// <summary>
+    /// Sender address (From header).
+    /// </summary>
+    [Required]
+    [MaxLength(512)]
+    [Column("FROM", TypeName = "nvarchar(512)")]
+#if NET
+    public string From { get; init; } = string.Empty;
+#else
+    public string From { get; set; } = string.Empty;
+#endif
+
+    /// <summary>
+    /// Recipient addresses (To header).
+    /// </summary>
+    [Column("TO", TypeName = "nvarchar(max)")]
+#if NET
+    public IEnumerable<string> To { get; init; } = [];
+#else
+    public IEnumerable<string> To { get; set; } = [];
+#endif
+
+    /// <summary>
+    /// CC addresses.
+    /// </summary>
+    [Column("CC", TypeName = "nvarchar(max)")]
+#if NET
+    public IEnumerable<string> Cc { get; init; } = [];
+#else
+    public IEnumerable<string> Cc { get; set; } = [];
+#endif
+
+    /// <summary>
+    /// Email subject.
+    /// </summary>
+    [MaxLength(998)]
+    [Column("SUBJECT", TypeName = "nvarchar(998)")]
+#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>
+    [Column("TEXT_BODY", TypeName = "nvarchar(max)")]
+#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>
+    [Column("HTML_BODY", TypeName = "nvarchar(max)")]
+#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>
+    [Column("DATE", TypeName = "datetime2")]
+#if NET
+    public DateTime Date { get; init; }
+#else
+    public DateTime Date { get; set; }
+#endif
+
+    /// <summary>
+    /// Whether the message has been marked as seen/read on the server.
+    /// </summary>
+    [Column("IS_SEEN", TypeName = "bit")]
+#if NET
+    public bool IsSeen { get; init; }
+#else
+    public bool IsSeen { get; set; }
+#endif
+
+    [ForeignKey(nameof(Account))]
+    [Column("ACCOUNT_ID", TypeName = "int")]
+#if NET
+    public int AccountId { get; init; }
+#else
+    public int AccountId { get; set; }
+#endif
+
+    /// <summary>
+    /// Attachments included with this message.
+    /// </summary>
+#if NET
+    public IEnumerable<EmailAttachment>? Attachments { get; init; }
+#else
+    public IEnumerable<EmailAttachment>? Attachments { get; set; }
+#endif
+
+#if NET
+    public EmailAccount? Account { get; init; }
+#else
+    public EmailAccount? Account { get; set; }
+#endif
+}