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.Infrastructure/Configuration/StreamTicketConfiguration.cs

@@ -0,0 +1,21 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class StreamTicketConfiguration
+    {
+        public const string SectionName = "StreamTicket";
+
+        [Required]
+        public required string PrivateKeyPath { get; set; }
+
+        [Required]
+        public string Audience { get; set; } = "media-gateway";
+
+        [Required]
+        public string Issuer { get; set; } = "musify-webapi";
+
+        [Range(60, 86400)]
+        public int TicketTtlSeconds { get; set; } = 3600;
+    }
+}