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/scripts/migrate.ps1

@@ -4,7 +4,7 @@
 param([Parameter(Mandatory)][string]$ConnectionString)
 
 $ErrorActionPreference = "Stop"
-$infraProject = Resolve-Path (Join-Path $PSScriptRoot "..\..\backend\Musify.Infrastructure")
+$infraProject = Resolve-Path (Join-Path $PSScriptRoot "..\..\backend\Core\Musify.Infrastructure")
 
 # The design-time factory reads the connection string from user-secrets.
 dotnet user-secrets set "Database:ConnectionString" $ConnectionString --project $infraProject | Out-Null