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.Api/DataTransferObjects/PlayLists/CreatePlayListRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record CreatePlayListRequest(
+    string Name,
+    string Description,
+    Guid? PictureIntentId);

backend/Musify.Api/DataTransferObjects/PlayLists/RequestPlayListPictureUploadRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record RequestPlayListPictureUploadRequest(
+    string FileType,
+    string ContentType,
+    long? ExpectedSizeBytes = null);

backend/Musify.Api/DataTransferObjects/PlayLists/UpdatePlayListRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record UpdatePlayListRequest(
+    string? NewName,
+    string? NewDescription,
+    Guid? NewPictureIntentId);