feat: add a group command

Application/Cqrs/Group/Commands/CreateGroup/CreateGroupCommand.cs

@@ -0,0 +1,9 @@
+using MediatR;
+
+namespace Mirea.Api.DataAccess.Application.Cqrs.Group.Commands.CreateGroup;
+
+public class CreateGroupCommand : IRequest<int>
+{
+    public required string Name { get; set; }
+    public int? FacultyId { get; set; }
+}

Application/Cqrs/Group/Commands/CreateGroup/CreateGroupCommandHandler.cs

@@ -0,0 +1,30 @@
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+using Mirea.Api.DataAccess.Application.Common.Exceptions;
+using Mirea.Api.DataAccess.Application.Interfaces.DbContexts.Schedule;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Mirea.Api.DataAccess.Application.Cqrs.Group.Commands.CreateGroup;
+
+public class CreateGroupCommandHandler(IGroupDbContext dbContext) : IRequestHandler<CreateGroupCommand, int>
+{
+    public async Task<int> Handle(CreateGroupCommand request, CancellationToken cancellationToken)
+    {
+        var entity = await dbContext.Groups.FirstOrDefaultAsync(g => g.Name == request.Name, cancellationToken: cancellationToken);
+
+        if (entity != null)
+            throw new RecordExistException(typeof(Domain.Schedule.Group), nameof(Domain.Schedule.Group.Name), entity.Id);
+
+        var group = new Domain.Schedule.Group()
+        {
+            Name = request.Name,
+            FacultyId = request.FacultyId
+        };
+
+        var result = await dbContext.Groups.AddAsync(group, cancellationToken);
+        await dbContext.SaveChangesAsync(cancellationToken);
+
+        return result.Entity.Id;
+    }
+}