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/PlayLists/CreatePlayListRequestValidator.cs

@@ -0,0 +1,18 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class CreatePlayListRequestValidator : AbstractValidator<CreatePlayListRequest>
+{
+    public CreatePlayListRequestValidator()
+    {
+        RuleFor(x => x.Name)
+            .NotEmpty()
+            .MaximumLength(50);
+
+        RuleFor(x => x.Description)
+            .NotEmpty()
+            .MaximumLength(256);
+    }
+}

Musify.Api/Validators/PlayLists/RequestPlayListPictureUploadRequestValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class RequestPlayListPictureUploadRequestValidator : AbstractValidator<RequestPlayListPictureUploadRequest>
+{
+    public RequestPlayListPictureUploadRequestValidator()
+    {
+        RuleFor(x => x.FileType)
+            .NotEmpty();
+
+        RuleFor(x => x.ContentType)
+            .NotEmpty();
+
+        RuleFor(x => x.ExpectedSizeBytes)
+            .GreaterThan(0)
+            .When(x => x.ExpectedSizeBytes.HasValue);
+    }
+}

Musify.Api/Validators/PlayLists/UpdatePlayListRequestValidator.cs

@@ -0,0 +1,18 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class UpdatePlayListRequestValidator : AbstractValidator<UpdatePlayListRequest>
+{
+    public UpdatePlayListRequestValidator()
+    {
+        RuleFor(x => x.NewName)
+            .MaximumLength(50)
+            .When(x => x.NewName is not null);
+
+        RuleFor(x => x.NewDescription)
+            .MaximumLength(256)
+            .When(x => x.NewDescription is not null);
+    }
+}