feat(domain): add all domain entities with legacy database mapping

Entities mapped to legacy database tables using [Table] and [Column] attributes:
- EmailAccount → TBDD_EMAIL_ACCOUNT (OAuth2 and password auth support)
- EmailProfile → TBEMLP_POLL_PROFILES (polling configuration)
- EmailProcess → TBEMLP_POLL_PROCESS (process definitions)
- ProcessStep → TBEMLP_POLL_STEPS (indexing steps)
- IndexingStep → TBEMLP_POLL_INDEXING_STEPS (DMS indexing fields)
- EmailHistory → TBEMLP_HISTORY (processed emails)
- EmailAttachment → TBEMLP_HISTORY_ATTACHMENT (email attachments)
- EmailOutbox → TBEMLP_EMAIL_OUT (outgoing email queue)

All entities follow Clean Architecture and DDD principles.
Database schema is read-only - no migrations will modify existing tables.
SNAKE_CASE columns mapped to PascalCase properties.

src/DigitalData.EmailProfiler.Domain/Entities/EmailProfile.cs

@@ -0,0 +1,88 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using DigitalData.EmailProfiler.Domain.Common;
+
+namespace DigitalData.EmailProfiler.Domain.Entities;
+
+[Table("TBEMLP_POLL_PROFILES")]
+public class EmailProfile : BaseEntity, IAggregateRoot
+{
+    [Key]
+    [Column("GUID", Order = 0)]
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    public int Id { get; set; }
+
+    [Column("PROFILE_NAME", TypeName = "varchar(100)")]
+    [Required]
+    [MaxLength(100)]
+    public string ProfileName { get; set; } = string.Empty;
+
+    [Column("POLL_TYPE", TypeName = "varchar(100)")]
+    [Required]
+    [MaxLength(100)]
+    public string PollType { get; set; } = "IMAP";
+
+    [Column("EMAIL_CONF_ID")]
+    [Required]
+    public int EmailAccountId { get; set; }
+
+    [Column("PROCESS_ID")]
+    public int? ProcessId { get; set; }
+
+    // Polling Configuration
+    [Column("POLL_INTERVAL")]
+    public int PollIntervalMinutes { get; set; } = 5;
+
+    [Column("LAST_TICK")]
+    public DateTime? LastPollTime { get; set; }
+
+    // Validation
+    [Column("VALIDATION_SQL", TypeName = "nvarchar(1024)")]
+    [MaxLength(1024)]
+    public string? ValidationSql { get; set; }
+
+    // Status
+    [Column("ACTIVE")]
+    public bool IsActive { get; set; }
+
+    [Column("SEQUENCE")]
+    public int? Sequence { get; set; }
+
+    [Column("COMMENT", TypeName = "varchar(500)")]
+    [MaxLength(500)]
+    public string? Comment { get; set; }
+
+    // Audit fields
+    [Column("ADDED_WHO", TypeName = "varchar(50)")]
+    [MaxLength(50)]
+    public string? AddedWho { get; set; }
+
+    [Column("ADDED_WHEN")]
+    public DateTime? AddedWhen { get; set; }
+
+    [Column("CHANGED_WHO", TypeName = "varchar(50)")]
+    [MaxLength(50)]
+    public string? ChangedWho { get; set; }
+
+    [Column("CHANGED_WHEN")]
+    public DateTime? ChangedWhen { get; set; }
+
+    // Navigation properties
+    [ForeignKey("EmailAccountId")]
+    public virtual EmailAccount? EmailAccount { get; set; }
+
+    [ForeignKey("ProcessId")]
+    public virtual EmailProcess? EmailProcess { get; set; }
+
+    public virtual ICollection<EmailHistory> EmailHistories { get; set; } = new List<EmailHistory>();
+
+    // Domain methods
+    public void UpdateLastPollTime() => LastPollTime = DateTime.UtcNow;
+
+    public bool ShouldPoll()
+    {
+        if (!IsActive) return false;
+        if (!LastPollTime.HasValue) return true;
+        return DateTime.UtcNow >= LastPollTime.Value.AddMinutes(PollIntervalMinutes);
+    }
+}