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.

backend/Musify.Api/Authentication/CurrentUser.cs

@@ -0,0 +1,43 @@
+using System.Reflection;
+using System.Security.Claims;
+
+namespace Musify.Api.Authentication;
+
+public sealed class CurrentUser : IBindableFromHttpContext<CurrentUser>
+{
+    private CurrentUser(ClaimsPrincipal principal)
+    {
+        Principal = principal;
+        var isAuthenticated = principal.Identity?.IsAuthenticated;
+        IsAuthenticated = isAuthenticated.HasValue && isAuthenticated.Value;
+
+        var rawId = principal.FindFirstValue(ClaimTypes.NameIdentifier);
+        if (long.TryParse(rawId, out var id) && id != 0)
+            Id = id;
+
+        Username = principal.FindFirstValue(ClaimTypes.Name);
+        FirstName = principal.FindFirstValue(ClaimTypes.GivenName);
+        LastName = principal.FindFirstValue(ClaimTypes.Surname);
+    }
+
+    public ClaimsPrincipal Principal { get; }
+
+    public bool IsAuthenticated { get; }
+
+    public long? Id { get; }
+
+    public string? Username { get; }
+
+    public string? FirstName { get; }
+
+    public string? LastName { get; }
+
+    public long RequiredId => Id ?? throw new InvalidOperationException("User Id claim is missing or invalid.");
+
+    public bool HasClaim(string type) => Principal.HasClaim(c => c.Type == type);
+
+    public string? FindClaim(string type) => Principal.FindFirstValue(type);
+
+    public static ValueTask<CurrentUser?> BindAsync(HttpContext context, ParameterInfo parameter) =>
+        ValueTask.FromResult<CurrentUser?>(new CurrentUser(context.User));
+}