Reorganize backend to mirror Musify.slnx folder structure

Moves the six backend projects on disk into Core/ and Hosts/
subfolders, matching the solution folders already defined in
Musify.slnx, so the file explorer reflects the same layout as the
solution:

  backend/Core/Musify.Domain
  backend/Core/Musify.Application
  backend/Core/Musify.Infrastructure
  backend/Hosts/Musify.Api
  backend/Hosts/Musify.Worker
  backend/Hosts/Musify.StreamingGateway

Updates every reference to the old paths: ProjectReference entries in
Musify.Api.csproj and Musify.Worker.csproj, the three Dockerfiles,
deploy/scripts/migrate.ps1|sh, deploy/zitadel/provision.ps1,
deploy/up.ps1, deploy/README.md and docs/development.md. Also fixes a
stale Dockerfile comment pointing at a compose file that no longer
exists.

Verified with `dotnet build Musify.slnx` against the new layout.

deploy/up.ps1

@@ -112,9 +112,9 @@ $appLines = if ($Apps) {
 } else {
 @"
   Run the apps from the IDE (or ./up.ps1 -Apps to run them in Docker):
-    dotnet run --project backend/Musify.Api               # :5111
-    dotnet run --project backend/Musify.StreamingGateway  # :8081
-    dotnet run --project backend/Musify.Worker            # background
+    dotnet run --project backend/Hosts/Musify.Api               # :5111
+    dotnet run --project backend/Hosts/Musify.StreamingGateway  # :8081
+    dotnet run --project backend/Hosts/Musify.Worker            # background
     npm --prefix web-player run dev                       # :5173
 "@
 }