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.Application/Users/Commands/CreateUserCommand.cs

@@ -0,0 +1,26 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Users.Responses;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Users.Commands
+{
+    public record CreateUserCommand(long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName)
+        : ICommand<Result<UserApplicationResponse>>
+    {
+        public static User ToEntity(CreateUserCommand command)
+        {
+            return new User
+            {
+                Id = command.Id,
+                Name = command.Name,
+                NormalizedName = command.Name.ToUpperInvariant(),
+                FirstName = command.FirstName,
+                SecondName = command.SecondName
+            };
+        }
+    }
+}

Musify.Application/Users/Commands/SyncUserCommand.cs

@@ -0,0 +1,13 @@
+using Ardalis.Result;
+using Mediator;
+
+namespace Musify.Application.Users.Commands
+{
+    public record SyncUserCommand(
+        long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName,
+        string? ProfilePictureUrl)
+        : ICommand<Result>;
+}