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/Scalar/ScalarExtensions.cs

@@ -0,0 +1,11 @@
+namespace Musify.Api.Scalar;
+
+public static class ScalarExtensions
+{
+    public static IServiceCollection AddScalarConfiguration(this IServiceCollection services)
+    {
+        services.ConfigureOptions<ScalarOptionsSetup>();
+
+        return services;
+    }
+}

Musify.Api/Scalar/ScalarOptionsSetup.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.Options;
+using Scalar.AspNetCore;
+using Musify.Api.Authentication;
+
+namespace Musify.Api.Scalar;
+
+public sealed class ScalarOptionsSetup(IOptions<AuthenticationConfiguration> options)
+    : IConfigureOptions<ScalarOptions>
+{
+    private const string SecuritySchemeId = "OAuth2";
+
+    private readonly AuthenticationConfiguration _configuration = options.Value;
+
+    public void Configure(ScalarOptions scalarOptions)
+    {
+        scalarOptions
+            .WithTitle("Musify API")
+            .AddPreferredSecuritySchemes(SecuritySchemeId)
+            .AddAuthorizationCodeFlow(SecuritySchemeId, flow =>
+            {
+                flow.ClientId = _configuration.ClientId;
+                flow.ClientSecret = _configuration.ClientSecret;
+                flow.AuthorizationUrl = _configuration.AuthorizationEndpoint;
+                flow.TokenUrl = _configuration.TokenEndpoint;
+                flow.Pkce = Pkce.Sha256;
+                flow.SelectedScopes = _configuration.Scopes;
+                flow.RedirectUri = _configuration.ScalarRedirectUri;
+                flow.RefreshUrl = _configuration.TokenEndpoint;
+            } )
+            .HideModels();
+    }
+}