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/Users/Queries/GetUserByIdQuery.cs

@@ -0,0 +1,9 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Users.Responses;
+
+namespace Musify.Application.Users.Queries
+{
+    public record GetUserByIdQuery(long Id)
+        : IQuery<Result<UserApplicationResponse> >;
+}

Musify.Application/Users/Queries/GetUsersQuery.cs

@@ -0,0 +1,10 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Pagination;
+using Musify.Application.Users.Responses;
+
+namespace Musify.Application.Users.Queries
+{
+    public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch = null)
+        : IQuery<Result<PaginatedResponse<UserApplicationResponse> >>;
+}