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/IndexingStep.cs

@@ -0,0 +1,56 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using DigitalData.EmailProfiler.Domain.Common;
+
+namespace DigitalData.EmailProfiler.Domain.Entities;
+
+[Table("TBEMLP_POLL_INDEXING_STEPS")]
+public class IndexingStep : BaseEntity
+{
+    [Key]
+    [Column("GUID", Order = 0)]
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    public int Id { get; set; }
+
+    [Column("STEP_ID")]
+    [Required]
+    public int StepId { get; set; }
+
+    [Column("INDEXNAME", TypeName = "varchar(100)")]
+    [Required]
+    [MaxLength(100)]
+    public string IndexName { get; set; } = string.Empty;
+
+    [Column("INDEXVALUE", TypeName = "varchar(100)")]
+    [Required]
+    [MaxLength(100)]
+    public string IndexValue { get; set; } = string.Empty;
+
+    [Column("ACTIVE")]
+    public bool IsActive { get; set; } = true;
+
+    [Column("USE_FOR_DIRECT_ANSWER")]
+    public bool UseForDirectAnswer { get; set; }
+
+    [Column("SEQUENCE")]
+    public int? Sequence { 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("StepId")]
+    public virtual ProcessStep? ProcessStep { get; set; }
+}