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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.StreamingGateway/Musify.StreamingGateway.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <UserSecretsId>media-gateway-2f1c9b6a-7d3e-4c0a-9a21-0b8e5d4c3f10</UserSecretsId>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Ardalis.Result" />
+    <PackageReference Include="Microsoft.IdentityModel.JsonWebTokens" />
+    <PackageReference Include="Yarp.ReverseProxy" />
+  </ItemGroup>
+
+</Project>