Add shared pagination, projections and ownership helpers; simplify query and command handlers

backend/Core/Musify.Application/Mixes/GetMixById.cs

@@ -3,6 +3,7 @@ using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
 using Musify.Application.Mixes.Responses;
+using Musify.Application.Shared;
 
 namespace Musify.Application.Mixes;
 
@@ -20,7 +21,7 @@ public class GetMixByIdQueryHandler(IDatabase database)
             .SingleOrDefaultAsync(mix => mix.Id == request.MixId && mix.UserId == request.UserId, cancellationToken);
 
         if (mix == null)
-            return Error.NotFound(description: $"Mix {request.MixId} was not found");
+            return AppErrors.NotFound("Mix", request.MixId);
 
         var orderedItems = mix.Items.OrderBy(item => item.Position).ToList();
         var items = await MixItemMapper.ToResponsesAsync(database, orderedItems, cancellationToken);