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.StreamingGateway/Configuration/StreamTicketValidationOptions.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.StreamingGateway.Configuration;
+
+public sealed class StreamTicketValidationOptions
+{
+    public const string SectionName = "StreamTicket";
+
+    [Required]
+    public required string PublicKeyPath { get; set; }
+
+    [Required]
+    public string Audience { get; set; } = "media-gateway";
+
+    [Required]
+    public string Issuer { get; set; } = "musify-webapi";
+
+    public string QueryParameterName { get; set; } = "t";
+
+    public string HeaderName { get; set; } = "X-Stream-Ticket";
+
+    public string MediaPathPrefix { get; set; } = "/media";
+}