feat(CountHistoryQuery): add CountHistoryQuery to retrieve envelope history count

- Introduced `CountHistoryQuery` record for querying the count of history entries of an envelope.
- Implemented `CountHistoryQueryHandler` using repository pattern and AutoMapper.
- Supports optional filtering by `Status`.
- Throws `NotFoundException` if necessary (placeholder for future extension).

EnvelopeGenerator.Application/Histories/Queries/CountHistoryQuery.cs

@@ -0,0 +1,54 @@
+using AutoMapper;
+using DigitalData.Core.Abstraction.Application.Repository;
+using DigitalData.Core.Exceptions;
+using EnvelopeGenerator.Application.Common.Dto.History;
+using EnvelopeGenerator.Domain.Constants;
+using MediatR;
+using System.ComponentModel.DataAnnotations;
+using EnvelopeGenerator.Domain.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.Histories.Queries;
+
+//TODO: Add sender query
+/// <summary>
+/// Repräsentiert eine Abfrage für die Verlaufshistorie eines Umschlags.
+/// </summary>
+public record CountHistoryQuery : HistoryQueryBase, IRequest<int>;
+
+/// <summary>
+/// 
+/// </summary>
+public class CountHistoryQueryHandler : IRequestHandler<CountHistoryQuery, int>
+{
+    private readonly IRepository<History> _repo;
+
+    private readonly IMapper _mapper;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repo"></param>
+    /// <param name="mapper"></param>
+    public CountHistoryQueryHandler(IRepository<History> repo, IMapper mapper)
+    {
+        _repo = repo;
+        _mapper = mapper;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    /// <exception cref="NotFoundException"></exception>
+    public Task<int> Handle(CountHistoryQuery request, CancellationToken cancel = default)
+    {
+        var query = _repo.Where(h => h.EnvelopeId == request.EnvelopeId);
+        if (request.Status is not null)
+            query = query.Where(h => h.Status == request.Status);
+
+        return query.CountAsync(cancel);
+    }
+}