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/Tracks/Queries/GetTrackByIdQuery.cs

@@ -0,0 +1,9 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Tracks.Responses;
+
+namespace Musify.Application.Tracks.Queries
+{
+    public record GetTrackByIdQuery(Guid TrackId)
+        : IQuery<Result<TrackApplicationResponse>>;
+}

Musify.Application/Tracks/Queries/GetTrackStreamQuery.cs

@@ -0,0 +1,9 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Tracks.Responses;
+
+namespace Musify.Application.Tracks.Queries
+{
+    public record GetTrackStreamQuery(Guid TrackId, long UserId)
+        : IQuery<Result<TrackStreamResponse>>;
+}

Musify.Application/Tracks/Queries/GetTracksByUserIdQuery.cs

@@ -0,0 +1,14 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Pagination;
+using Musify.Application.Tracks.Responses;
+
+namespace Musify.Application.Tracks.Queries
+{
+    public record GetTracksByUserIdQuery(
+        long UserId,
+    string? Name = null,
+        int PageNumber = 1,
+        int PageSize = 10)
+        : IQuery<Result<PaginatedResponse<TrackApplicationResponse> > >;
+}

Musify.Application/Tracks/Queries/GetTracksQuery.cs

@@ -0,0 +1,10 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.Pagination;
+using Musify.Application.Tracks.Responses;
+
+namespace Musify.Application.Tracks.Queries
+{
+    public record GetTracksQuery(int PageNumber = 1, int PageSize = 10)
+        : IQuery<Result<PaginatedResponse<TrackApplicationResponse> >>;
+}