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.Worker/Musify.Worker.csproj

@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk.Worker">
+
+  <PropertyGroup>
+    <UserSecretsId>dotnet-PictureWorker-1dfff89f-0602-41b7-8eaf-4da2c3358789</UserSecretsId>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Ardalis.Result" />
+    <PackageReference Include="AWSSDK.S3" />
+    <PackageReference Include="DispatchR.Mediator" />
+    <PackageReference Include="MassTransit" />
+    <PackageReference Include="MassTransit.RabbitMQ" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.Relational" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" />
+    <PackageReference Include="Microsoft.Extensions.Options.DataAnnotations" />
+    <PackageReference Include="MimeMapping" />
+    <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" />
+    <PackageReference Include="SixLabors.ImageSharp" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Musify.Application\Musify.Application.csproj" />
+    <ProjectReference Include="..\Musify.Domain\Musify.Domain.csproj" />
+    <ProjectReference Include="..\Musify.Infrastructure\Musify.Infrastructure.csproj" />
+  </ItemGroup>
+
+</Project>