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/zitadel/provision.ps1

@@ -28,7 +28,7 @@ param(
 $ErrorActionPreference = "Stop"
 $deployDir  = Split-Path $PSScriptRoot -Parent
 $repoDir    = Split-Path $deployDir -Parent
-$apiProject = Join-Path $repoDir "backend\Musify.Api"
+$apiProject = Join-Path $repoDir "backend\Hosts\Musify.Api"
 $webEnvFile = Join-Path $repoDir "web-player\.env"
 $envFile    = if ($EnvFile) { $EnvFile } else { Join-Path $deployDir ".env" }
 $patFile    = Join-Path $PSScriptRoot "$OutputDir\admin-sa.pat"