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.slnx

@@ -0,0 +1,12 @@
+<Solution>
+  <Folder Name="/Core/">
+    <Project Path="Musify.Domain/Musify.Domain.csproj" />
+    <Project Path="Musify.Application/Musify.Application.csproj" />
+    <Project Path="Musify.Infrastructure/Musify.Infrastructure.csproj" />
+  </Folder>
+  <Folder Name="/Hosts/">
+    <Project Path="Musify.Api/Musify.Api.csproj" />
+    <Project Path="Musify.Worker/Musify.Worker.csproj" />
+    <Project Path="Musify.StreamingGateway/Musify.StreamingGateway.csproj" />
+  </Folder>
+</Solution>