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.Application/Configuration/StreamGatewayConfiguration.cs

@@ -0,0 +1,16 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Application.Configuration
+{
+    public class StreamGatewayConfiguration
+    {
+        public const string SectionName = "StreamGateway";
+
+        [Url]
+        [Required]
+        public required string PublicBaseUrl { get; set; }
+
+        [Required]
+        public string AudioFileName { get; set; } = "audio.m4a";
+    }
+}