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/JwtBearerOptionsSetup.cs

@@ -0,0 +1,37 @@
+using Microsoft.AspNetCore.Authentication.JwtBearer;
+using Microsoft.Extensions.Options;
+using Microsoft.IdentityModel.Tokens;
+using System.Security.Claims;
+
+namespace Musify.Api.Authentication;
+
+public sealed class JwtBearerOptionsSetup(IOptions<AuthenticationConfiguration> options)
+    : IConfigureNamedOptions<JwtBearerOptions>
+{
+    private readonly AuthenticationConfiguration _configuration = options.Value;
+
+    public void Configure(string? name, JwtBearerOptions options)
+    {
+        if (name != JwtBearerDefaults.AuthenticationScheme)
+        {
+            return;
+        }
+
+        Configure(options);
+    }
+
+    public void Configure(JwtBearerOptions options)
+    {
+        options.MetadataAddress = _configuration.MetadataAddress;
+        options.RequireHttpsMetadata = _configuration.RequireHttpsMetadata;
+        options.Audience = _configuration.AudienceAddress;
+        options.TokenValidationParameters = new TokenValidationParameters
+        {
+            ValidIssuer = _configuration.IssuerAddress,
+            ValidAudience = _configuration.AudienceAddress,
+            RoleClaimType = ClaimTypes.Role,
+            NameClaimType = ClaimTypes.Name,
+        };
+        options.EventsType = typeof(JwtBearerEventsHandler);
+    }
+}