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.Application/Configuration/StreamGatewayConfiguration.cs

@@ -0,0 +1,16 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Application.Configuration
+{
+    public class StreamGatewayConfiguration
+    {
+        public const string SectionName = "StreamGateway";
+
+        [Url]
+        [Required]
+        public required string PublicBaseUrl { get; set; }
+
+        [Required]
+        public string AudioFileName { get; set; } = "audio.m4a";
+    }
+}