feat(application): add MediatR commands and handlers with exception improvements

MediatR Commands (CQRS Pattern):
- CreateEmailProfileCommand + Handler
- UpdateEmailProfileCommand + Handler
- DeleteEmailProfileCommand + Handler
- CreateEmailAccountCommand + Handler
- ProcessEmailCommand + Handler (core email processing logic)

Command Handlers:
- Create/Update/Delete operations for EmailProfile
- Create operation for EmailAccount
- ProcessEmail: Complete email processing workflow including:
  * Duplicate detection using MessageId hash
  * Email history creation
  * PDF attachment validation
  * windream DMS archiving support (placeholder)
  * Domain event publishing (EmailProcessedEvent)
  * Error handling and status tracking

Exception Improvements:
- Added ErrorCode property to DomainException
- Added ErrorCode overload to ValidationException
- Simplified AttachmentProcessingException to use base ErrorCode

Field Mappings Fixed:
- EmailProfile: ProcessId (not EmailProcessId), ValidationSql (not SenderFilter/SubjectFilter)
- EmailAccount: Username, EncryptedPassword, UseOAuth2, EncryptedClientSecret
- EmailHistory: SenderAddress, EmailDate, OriginalMessageId, EmailBodyText/Html
- EmailAttachment: OriginalFileName, SavedFileName, FilePath, FileSize
- Audit fields: AddedWhen/AddedWho, ChangedWhen/ChangedWho (not CreatedDate/By, ModifiedDate/By)

All commands follow Clean Architecture and use UnitOfWork pattern.
Build successful with 1 minor warning (dmsService marked for future implementation).

src/DigitalData.EmailProfiler.Application/Features/EmailProfiles/Commands/CreateEmailProfileCommand.cs

@@ -0,0 +1,16 @@
+using MediatR;
+
+namespace DigitalData.EmailProfiler.Application.Features.EmailProfiles.Commands;
+
+/// <summary>
+/// Command to create a new email profile.
+/// </summary>
+public record CreateEmailProfileCommand : IRequest<int>
+{
+    public string ProfileName { get; init; } = string.Empty;
+    public int EmailAccountId { get; init; }
+    public int? ProcessId { get; init; }
+    public string? ValidationSql { get; init; }
+    public int PollIntervalMinutes { get; init; } = 15;
+    public bool IsActive { get; init; } = true;
+}

src/DigitalData.EmailProfiler.Application/Features/EmailProfiles/Commands/CreateEmailProfileCommandHandler.cs

@@ -0,0 +1,29 @@
+using DigitalData.EmailProfiler.Application.Interfaces.Repositories;
+using DigitalData.EmailProfiler.Domain.Entities;
+using MediatR;
+
+namespace DigitalData.EmailProfiler.Application.Features.EmailProfiles.Commands;
+
+public class CreateEmailProfileCommandHandler(IUnitOfWork unitOfWork) 
+    : IRequestHandler<CreateEmailProfileCommand, int>
+{
+    public async Task<int> Handle(CreateEmailProfileCommand request, CancellationToken cancellationToken)
+    {
+        var profile = new EmailProfile
+        {
+            ProfileName = request.ProfileName,
+            EmailAccountId = request.EmailAccountId,
+            ProcessId = request.ProcessId,
+            ValidationSql = request.ValidationSql,
+            PollIntervalMinutes = request.PollIntervalMinutes,
+            IsActive = request.IsActive,
+            AddedWhen = DateTime.Now,
+            AddedWho = "System" // TODO: Get from current user context
+        };
+
+        var createdProfile = await unitOfWork.EmailProfiles.AddAsync(profile, cancellationToken);
+        await unitOfWork.SaveChangesAsync(cancellationToken);
+
+        return createdProfile.Id;
+    }
+}

src/DigitalData.EmailProfiler.Application/Features/EmailProfiles/Commands/DeleteEmailProfileCommand.cs

@@ -0,0 +1,8 @@
+using MediatR;
+
+namespace DigitalData.EmailProfiler.Application.Features.EmailProfiles.Commands;
+
+/// <summary>
+/// Command to delete an email profile.
+/// </summary>
+public record DeleteEmailProfileCommand(int Id) : IRequest<Unit>;

src/DigitalData.EmailProfiler.Application/Features/EmailProfiles/Commands/DeleteEmailProfileCommandHandler.cs

@@ -0,0 +1,20 @@
+using DigitalData.EmailProfiler.Application.Interfaces.Repositories;
+using DigitalData.EmailProfiler.Domain.Exceptions;
+using MediatR;
+
+namespace DigitalData.EmailProfiler.Application.Features.EmailProfiles.Commands;
+
+public class DeleteEmailProfileCommandHandler(IUnitOfWork unitOfWork) 
+    : IRequestHandler<DeleteEmailProfileCommand, Unit>
+{
+    public async Task<Unit> Handle(DeleteEmailProfileCommand request, CancellationToken cancellationToken)
+    {
+        var profile = await unitOfWork.EmailProfiles.GetByIdAsync(request.Id, cancellationToken)
+            ?? throw new DomainException($"Email profile with ID {request.Id} not found");
+
+        await unitOfWork.EmailProfiles.DeleteAsync(profile, cancellationToken);
+        await unitOfWork.SaveChangesAsync(cancellationToken);
+
+        return Unit.Value;
+    }
+}

src/DigitalData.EmailProfiler.Application/Features/EmailProfiles/Commands/UpdateEmailProfileCommand.cs

@@ -0,0 +1,15 @@
+using MediatR;
+
+namespace DigitalData.EmailProfiler.Application.Features.EmailProfiles.Commands;
+
+/// <summary>
+/// Command to update an existing email profile.
+/// </summary>
+public record UpdateEmailProfileCommand : IRequest<Unit>
+{
+    public int Id { get; init; }
+    public string ProfileName { get; init; } = string.Empty;
+    public string? ValidationSql { get; init; }
+    public int PollIntervalMinutes { get; init; }
+    public bool IsActive { get; init; }
+}

src/DigitalData.EmailProfiler.Application/Features/EmailProfiles/Commands/UpdateEmailProfileCommandHandler.cs

@@ -0,0 +1,27 @@
+using DigitalData.EmailProfiler.Application.Interfaces.Repositories;
+using DigitalData.EmailProfiler.Domain.Exceptions;
+using MediatR;
+
+namespace DigitalData.EmailProfiler.Application.Features.EmailProfiles.Commands;
+
+public class UpdateEmailProfileCommandHandler(IUnitOfWork unitOfWork) 
+    : IRequestHandler<UpdateEmailProfileCommand, Unit>
+{
+    public async Task<Unit> Handle(UpdateEmailProfileCommand request, CancellationToken cancellationToken)
+    {
+        var profile = await unitOfWork.EmailProfiles.GetByIdAsync(request.Id, cancellationToken)
+            ?? throw new DomainException($"Email profile with ID {request.Id} not found");
+
+        profile.ProfileName = request.ProfileName;
+        profile.ValidationSql = request.ValidationSql;
+        profile.PollIntervalMinutes = request.PollIntervalMinutes;
+        profile.IsActive = request.IsActive;
+        profile.ChangedWhen = DateTime.Now;
+        profile.ChangedWho = "System"; // TODO: Get from current user context
+
+        await unitOfWork.EmailProfiles.UpdateAsync(profile, cancellationToken);
+        await unitOfWork.SaveChangesAsync(cancellationToken);
+
+        return Unit.Value;
+    }
+}