refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

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));
+}