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.

docs/development.md

@@ -20,9 +20,9 @@ Otros modificadores: `-Tools` (pgAdmin), `-SkipMigrations`, `-Down`, `-Destroy`.
 ## Aplicaciones
 
 ```powershell
-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
 ```
 
@@ -50,7 +50,7 @@ No hay migración automática al arrancar; `up.ps1` las aplica en el bootstrap.
 A mano, tras cambios de esquema:
 
 ```powershell
-dotnet ef database update --project backend/Musify.Infrastructure --startup-project backend/Musify.Infrastructure --context Database
+dotnet ef database update --project backend/Core/Musify.Infrastructure --startup-project backend/Core/Musify.Infrastructure --context Database
 ```
 
 `Musify.Infrastructure` es a la vez el proyecto de migraciones y el de arranque: