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

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Musify.Application/Shared/ImageSize.cs

@@ -0,0 +1,7 @@
+namespace Musify.Application.Shared
+{
+    public record ImageSize(
+        string SavePath,
+        int Width,
+        int Height);
+}

Musify.Application/Shared/StorageKey.cs

@@ -0,0 +1,12 @@
+namespace Musify.Application.Shared
+{
+    public static class StorageKey
+    {
+        public static string Combine(params string[] segments)
+        {
+            return string.Join('/', segments
+                .Where(static s => !string.IsNullOrWhiteSpace(s))
+                .Select(static s => s.Trim().Trim('/', '\\')));
+        }
+    }
+}