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

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

@@ -33,7 +33,7 @@ public class GenerateMixesForUserCommandHandler(
         if (libraryTrackIds.Count == 0)
         {
             logger.LogInformation("User {UserId} has no tracks yet, skipping mix generation", userId);
-            return new Success();
+            return Result.Success;
         }
 
         var historyTrackIds = await database.ListeningHistories
@@ -69,23 +69,15 @@ public class GenerateMixesForUserCommandHandler(
         if (drafts.Count == 0)
         {
             logger.LogInformation("No candidate songs found for user {UserId}, skipping mix generation", userId);
-            return new Success();
+            return Result.Success;
         }
 
         await ReplaceMixesAsync(userId, drafts, cancellationToken);
 
-        try
-        {
-            await database.SaveChangesAsync(cancellationToken);
-        }
-        catch (Exception exception)
-        {
-            logger.LogError(exception, "Failed to generate mixes for user {UserId}", userId);
-            return Error.Failure(description: $"Failed to generate mixes for user {userId}");
-        }
+        await database.SaveChangesAsync(cancellationToken);
 
         logger.LogInformation("Generated {Count} mixes for user {UserId}", drafts.Count, userId);
-        return new Success();
+        return Result.Success;
     }
 
     private List<Guid> Take(IEnumerable<Guid> trackIds) =>

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);

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

@@ -1,4 +1,3 @@
-using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
@@ -7,18 +6,19 @@ using Musify.Application.Mixes.Responses;
 namespace Musify.Application.Mixes;
 
 public record GetMixesByUserIdQuery(long UserId)
-    : IQuery<ErrorOr<IReadOnlyList<MixApplicationResponse>>>;
+    : IQuery<IReadOnlyList<MixApplicationResponse>>;
 
 public class GetMixesByUserIdQueryHandler(IDatabase database)
-    : IQueryHandler<GetMixesByUserIdQuery, ErrorOr<IReadOnlyList<MixApplicationResponse>>>
+    : IQueryHandler<GetMixesByUserIdQuery, IReadOnlyList<MixApplicationResponse>>
 {
-    public async ValueTask<ErrorOr<IReadOnlyList<MixApplicationResponse>>> Handle(GetMixesByUserIdQuery request, CancellationToken cancellationToken)
+    public async ValueTask<IReadOnlyList<MixApplicationResponse>> Handle(GetMixesByUserIdQuery request, CancellationToken cancellationToken)
     {
         var mixes = await database.Mixes
             .AsNoTracking()
             .Where(mix => mix.UserId == request.UserId)
             .Include(mix => mix.Items)
             .OrderBy(mix => mix.Position)
+            .ThenBy(mix => mix.Id)
             .ToListAsync(cancellationToken);
 
         var orderedItemsByMix = mixes.ToDictionary(