chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

backend/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();
+    }
+}