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/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;
+}