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.Api/Endpoints/UserEndpoints.cs

@@ -0,0 +1,67 @@
+using Mediator;
+using Musify.Application.Users.Commands;
+using Musify.Application.Users.Queries;
+using Musify.Api.DataTransferObjects.Users;
+using Musify.Api.Extensions;
+using Musify.Api.Filters;
+
+namespace Musify.Api.Endpoints;
+
+public static class UserEndpoints
+{
+    public static IEndpointRouteBuilder MapUserEndpoints(this IEndpointRouteBuilder app)
+    {
+        var group = app.MapGroup("/users")
+            .WithTags("Users");
+
+        group.MapGet("/", GetUsers)
+            .WithName("GetUsers")
+            .WithSummary("Get Paginated Users.");
+
+        group.MapGet("/{id}", GetUserById)
+            .WithName("GetUserById")
+            .WithSummary("Get A User By Id.");
+
+        group.MapPost("/", CreateUser)
+            .WithName("CreateUser")
+            .WithSummary("Create A New User.")
+            .AddEndpointFilter<ValidationFilter<CreateUserRequest>>();
+
+        return app;
+    }
+
+    private static async Task<IResult> GetUsers(
+        IMediator mediator,
+        CancellationToken cancellationToken,
+        string? usernameSearch,
+        int pageNumber = 1,
+        int pageSize = 10)
+    {
+        var result = await mediator.Send(new GetUsersQuery(pageNumber, pageSize, usernameSearch), cancellationToken);
+        return result.ToHttpResult();
+    }
+
+    private static async Task<IResult> GetUserById(
+        IMediator mediator,
+        long id,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new GetUserByIdQuery(id), cancellationToken);
+        return result.ToHttpResult();
+    }
+
+    private static async Task<IResult> CreateUser(
+        IMediator mediator,
+        CreateUserRequest request,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(
+            new CreateUserCommand(request.Id, request.Name, request.FirstName, request.SecondName),
+            cancellationToken);
+
+        if (!result.IsSuccess)
+            return result.ToHttpResult();
+
+        return Results.Created($"/users/{result.Value.Id}", result.Value);
+    }
+}