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

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Musify.Api/Validators/Tracks/RequestTrackUploadUrlsRequestValidator.cs

@@ -0,0 +1,30 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Tracks;
+
+namespace Musify.Api.Validators.Tracks;
+
+public sealed class RequestTrackUploadUrlsRequestValidator : AbstractValidator<RequestTrackUploadUrlsRequest>
+{
+    public RequestTrackUploadUrlsRequestValidator()
+    {
+        RuleFor(x => x.PictureFileType)
+            .NotEmpty();
+
+        RuleFor(x => x.PictureContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.AudioFileType)
+            .NotEmpty();
+
+        RuleFor(x => x.AudioContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.ExpectedPictureSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedPictureSizeBytes.HasValue);
+
+        RuleFor(x => x.ExpectedAudioSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedAudioSizeBytes.HasValue);
+    }
+}