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.Infrastructure/MassTransit/RoutingSlip/RoutingSlipVariableNames.cs

@@ -0,0 +1,25 @@
+namespace Musify.Infrastructure.MassTransit
+{
+    internal static class RoutingSlipVariableNames
+    {
+        internal static class Workflow
+        {
+            internal const string TemporalDirectory = "Workflow.TemporalDirectory";
+            internal const string CorrelationId = "Workflow.CorrelationId";
+        }
+
+        internal static class Audio
+        {
+            internal const string SourceFilePath = "Audio.SourceFilePath";
+            internal const string TranscodedDirectory = "Audio.TranscodedDirectory";
+        }
+
+        internal static class Picture
+        {
+            internal const string OriginalFilePath = "Picture.OriginalFilePath";
+            internal const string SmallResizedFilePath = "Picture.SmallResizedFilePath";
+            internal const string MediumResizedFilePath = "Picture.MediumResizedFilePath";
+            internal const string LargeResizedFilePath = "Picture.LargeResizedFilePath";
+        }
+    }
+}