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.StreamingGateway/Configuration/StreamTicketValidationOptions.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.StreamingGateway.Configuration;
+
+public sealed class StreamTicketValidationOptions
+{
+    public const string SectionName = "StreamTicket";
+
+    [Required]
+    public required string PublicKeyPath { get; set; }
+
+    [Required]
+    public string Audience { get; set; } = "media-gateway";
+
+    [Required]
+    public string Issuer { get; set; } = "musify-webapi";
+
+    public string QueryParameterName { get; set; } = "t";
+
+    public string HeaderName { get; set; } = "X-Stream-Ticket";
+
+    public string MediaPathPrefix { get; set; } = "/media";
+}