Add RemoveEnvelopeReceiverCommand and refactor controller

Introduced `RemoveEnvelopeReceiverCommand` and its handler to
facilitate the removal of envelope receivers. Updated the
`EnvelopeReceiverController` to use C# 9's record-like syntax
for dependency injection, eliminating explicit private fields.
Added a `RemoveAsync` method to handle DELETE requests for
removing envelope receivers. Enhanced code documentation with
additional XML comments.

EnvelopeGenerator.Application/EnvelopeReceivers/Commands/RemoveEnvelopeReceiverCommand.cs

@@ -0,0 +1,84 @@
+using DigitalData.Core.Abstraction.Application.Repository;
+using DigitalData.Core.Exceptions;
+using EnvelopeGenerator.Domain.Constants;
+using EnvelopeGenerator.Domain.Entities;
+using EnvelopeGenerator.Domain.Exceptions;
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.EnvelopeReceivers.Commands;
+
+/// <summary>
+/// 
+/// </summary>
+public record RemoveEnvelopeReceiverCommand : IRequest
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public required int EnvelopeId { get; init; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public required int ReceiverId { get; init; }
+
+    internal int? UserId { get; private set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="userId"></param>
+    /// <returns></returns>
+    public RemoveEnvelopeReceiverCommand Authenticated(int userId)
+    {
+        UserId = userId;
+        return this;
+    }
+}
+
+/// <summary>
+/// 
+/// </summary>
+/// <param name="Repo"></param>
+public class RemoveEnvelopeReceiverCommandHandler(
+    IRepository<EnvelopeReceiver> Repo,
+    IRepository<DocReceiverElement> ElementRepo,
+    IRepository<ElementAnnotation> AnnotRepo) : IRequestHandler<RemoveEnvelopeReceiverCommand>
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    /// <exception cref="NotImplementedException"></exception>
+    public async Task Handle(RemoveEnvelopeReceiverCommand request, CancellationToken cancel)
+    {
+        var er = await Repo.Query
+            .Where(er => er.EnvelopeId == request.EnvelopeId && er.ReceiverId == request.ReceiverId)
+            .Include(er => er.Envelope)
+            .SingleOrDefaultAsync(cancel)
+                ?? throw new NotFoundException();
+
+        if (er.Envelope is null)
+            throw new InvalidOperationException($"In {nameof(RemoveEnvelopeReceiverCommandHandler)} EnvelopeReceiver.Envelope is null." +
+                $"This should not happen." +
+                $"EnvelopeId: {request.EnvelopeId}, ReceiverId: {request.ReceiverId}");
+
+        if (request.UserId is int userId && er.Envelope.UserId != userId)
+            throw new AuthorizationException("Cannot remove receiver from envelope that you do not own.");
+
+        if (er.Envelope.Status > EnvelopeStatus.EnvelopeSaved)
+            throw new ForbiddenException("Cannot remove receiver from envelope that is already processed.");
+
+        // 1. Delete all ElementAnnotations for this EnvelopeReceiver
+        await AnnotRepo.DeleteAsync(ann => ann.Element.Document.EnvelopeId == request.EnvelopeId && ann.Element.ReceiverId == request.ReceiverId, cancel);
+
+        // 2. Delete all DocReceiverElements for this EnvelopeReceiver
+        await ElementRepo.DeleteAsync(el => el.Document.EnvelopeId == request.EnvelopeId && el.ReceiverId == request.ReceiverId, cancel);
+
+        // 3. Delete the EnvelopeReceiver
+        await Repo.DeleteAsync(er => er.EnvelopeId == request.EnvelopeId && er.ReceiverId == request.ReceiverId, cancel);
+    }
+}