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/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;
 }