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/Authentication/AuthenticationConfiguration.cs

@@ -0,0 +1,39 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Api.Authentication;
+
+public sealed class AuthenticationConfiguration
+{
+    public const string SectionName = "Authentication";
+
+    [Required]
+    [Url]
+    public required string MetadataAddress { get; set; }
+
+    [Required]
+    public required string IssuerAddress { get; set; }
+
+    [Required]
+    public required string AudienceAddress { get; set; }
+
+    [Required]
+    public required string ClientId { get; set; }
+
+    public string? ClientSecret { get; set; }
+
+    [Required]
+    [Url]
+    public required string AuthorizationEndpoint { get; set; }
+
+    [Required]
+    [Url]
+    public required string TokenEndpoint { get; set; }
+
+    [Required]
+    [MinLength(1)]
+    public required string[] Scopes { get; set; }
+
+    public string? ScalarRedirectUri { get; set; }
+
+    public bool RequireHttpsMetadata { get; set; } = true;
+}