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

backend/Core/Musify.Application/Users/GetUserProfile.cs

@@ -1,26 +1,14 @@
-using System.Text.Json.Serialization;
 using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
-using Musify.Application.Serialization;
+using Musify.Application.Shared;
+using Musify.Application.Users.Responses;
 
 namespace Musify.Application.Users;
 
 public record GetUserProfileQuery(long UserId, long? ViewerId = null) : IQuery<ErrorOr<UserProfileResponse>>;
 
-public record UserProfileResponse(
-    [property: JsonConverter(typeof(LongAsStringConverter))] long Id,
-    string Name,
-    string? FirstName,
-    string? SecondName,
-    string? ProfilePictureUrl,
-    DateTime CreatedAt,
-    int FollowersCount,
-    int FollowingCount,
-    bool IsFollowing,
-    bool CanViewFollowers);
-
 public class GetUserProfileQueryHandler(IDatabase database)
     : IQueryHandler<GetUserProfileQuery, ErrorOr<UserProfileResponse>>
 {
@@ -41,7 +29,7 @@ public class GetUserProfileQueryHandler(IDatabase database)
             .SingleOrDefaultAsync(cancellationToken);
 
         if (user == null)
-            return Error.NotFound();
+            return AppErrors.NotFound("User", request.UserId);
 
         var followersCount = await database.UserFollows
             .AsNoTracking()