Add recent receiver suggestions query and DTO

Introduce `RecentReceiverSuggestionDto` to represent recent receiver suggestions with `FullName` and `EmailAddress`.

Add `ReadRecentReceiverSuggestionsQuery` to retrieve recent receiver suggestions scoped by sender username and limited to 3-5 results.

Implement `ReadRecentReceiverSuggestionsQueryHandler` to process the query using Entity Framework Core and AutoMapper.

Update `MappingProfile` to map `EnvelopeReceiver` to `RecentReceiverSuggestionDto`.

EnvelopeGenerator.Application/EnvelopeReceivers/Queries/ReadRecentReceiverSuggestionsQuery.cs

@@ -0,0 +1,72 @@
+using AutoMapper;
+using DigitalData.Core.Abstraction.Application.Repository;
+using EnvelopeGenerator.Application.Common.Dto.Receiver;
+using EnvelopeGenerator.Domain.Entities;
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.EnvelopeReceivers.Queries;
+
+/// <summary>
+/// Reads recent receiver suggestions for envelope creation.
+/// </summary>
+/// <remarks>
+/// Sender scope is assigned via chain method to avoid exposing username mutation from API query binding.
+/// </remarks>
+public record ReadRecentReceiverSuggestionsQuery : IRequest<IReadOnlyList<RecentReceiverSuggestionDto>>
+{
+    /// <summary>
+    /// Sender username used for scoping suggestion results.
+    /// </summary>
+    internal string? Username { get; private init; }
+
+    /// <summary>
+    /// Desired number of results. Effective range is clamped to 3..5.
+    /// </summary>
+    public int Take { get; init; } = 5;
+
+    /// <summary>
+    /// Returns a new query instance scoped to the specified sender username.
+    /// </summary>
+    /// <param name="username">Sender username from authenticated claims.</param>
+    /// <returns>Scoped query instance.</returns>
+    public ReadRecentReceiverSuggestionsQuery ForSender(string username)
+    {
+        return this with { Username = username };
+    }
+}
+
+/// <summary>
+/// Handles <see cref="ReadRecentReceiverSuggestionsQuery"/>.
+/// </summary>
+public class ReadRecentReceiverSuggestionsQueryHandler(IRepository<EnvelopeReceiver> repository, IMapper mapper)
+    : IRequestHandler<ReadRecentReceiverSuggestionsQuery, IReadOnlyList<RecentReceiverSuggestionDto>>
+{
+    /// <summary>
+    /// Retrieves distinct recent receiver suggestions grouped by receiver email.
+    /// </summary>
+    /// <param name="request">Suggestion query.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Ordered list of recent receiver suggestions.</returns>
+    public async Task<IReadOnlyList<RecentReceiverSuggestionDto>> Handle(ReadRecentReceiverSuggestionsQuery request, CancellationToken cancellationToken)
+    {
+        var take = Math.Clamp(request.Take, 3, 5);
+
+        var query = repository.Query
+            .AsNoTracking()
+            .Include(er => er.Receiver)
+            .Where(er => er.Receiver != null);
+
+        if (request.Username is not null)
+            query = query.Where(er => er.Envelope != null && er.Envelope.User.Username == request.Username);
+
+        var rows = await query
+            .GroupBy(er => er.Receiver!.EmailAddress.ToUpperInvariant())
+            .Select(g => g.OrderByDescending(er => er.ChangedWhen ?? er.AddedWhen).First())
+            .OrderByDescending(er => er.ChangedWhen ?? er.AddedWhen)
+            .Take(take)
+            .ToListAsync(cancellationToken);
+
+        return mapper.Map<List<RecentReceiverSuggestionDto>>(rows);
+    }
+}