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.Application/Musify.Application.csproj

@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <PackageReference Include="Ardalis.Result" />
+    <PackageReference Include="MassTransit" />
+    <PackageReference Include="Mediator.Abstractions" />
+    <PackageReference Include="Mediator.SourceGenerator">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.EntityFrameworkCore" />
+    <PackageReference Include="Microsoft.Extensions.Configuration" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" />
+    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" />
+    <PackageReference Include="Microsoft.Extensions.Options.DataAnnotations" />
+    <PackageReference Include="X.PagedList" />
+    <PackageReference Include="X.PagedList.EF" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Musify.Domain\Musify.Domain.csproj" />
+  </ItemGroup>
+
+</Project>