refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Musify.Application/PlayLists/Handlers/GetPlayListByIdQueryHandler.cs

@@ -0,0 +1,26 @@
+using Ardalis.Result;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.PlayLists.Responses;
+using Musify.Application.PlayLists.Queries;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.PlayLists.Handlers
+{
+    public class GetPlayListByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListByIdQuery, Result<PlayListApplicationResponse>>
+    {
+        public async ValueTask<Result<PlayListApplicationResponse>> Handle(GetPlayListByIdQuery request, CancellationToken cancellationToken)
+        {
+            var playList = await database.PlayLists
+                .AsNoTracking()
+                .Where(p => p.Id == request.Id)
+                .Select(p => PlayListApplicationResponse.FromEntity(p))
+                .SingleOrDefaultAsync(cancellationToken);
+
+            return playList is null
+                ? Result<PlayListApplicationResponse>.NotFound()
+                : Result.Success(playList);
+        }
+    }
+}