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/Users/CreateUserRequestValidator.cs

@@ -0,0 +1,25 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Users;
+
+namespace Musify.Api.Validators.Users;
+
+public sealed class CreateUserRequestValidator : AbstractValidator<CreateUserRequest>
+{
+    public CreateUserRequestValidator()
+    {
+        RuleFor(x => x.Id)
+            .NotEmpty();
+
+        RuleFor(x => x.Name)
+            .NotEmpty()
+            .MaximumLength(48);
+
+        RuleFor(x => x.FirstName)
+            .MaximumLength(48)
+            .When(x => x.FirstName is not null);
+
+        RuleFor(x => x.SecondName)
+            .MaximumLength(48)
+            .When(x => x.SecondName is not null);
+    }
+}