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

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

@@ -19,13 +19,13 @@ public class UnfollowUserCommandHandler(
             .SingleOrDefaultAsync(f => f.FollowerId == request.FollowerId && f.FollowedId == request.FollowedId, cancellationToken);
 
         if (follow == null)
-            return new Success();
+            return Result.Success;
 
         database.UserFollows.Remove(follow);
         await database.SaveChangesAsync(cancellationToken);
 
         logger.LogInformation("User {FollowerId} unfollowed user {FollowedId}", request.FollowerId, request.FollowedId);
 
-        return new Success();
+        return Result.Success;
     }
 }