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.sh

@@ -18,7 +18,7 @@ CONN="Host=postgres;Port=5432;Database=$(env_get "$ENV_FILE" POSTGRES_DB);Userna
 docker run --rm \
     --network "$NETWORK" \
     -v "$BACKEND_DIR":/src \
-    -w /src/Musify.Infrastructure \
+    -w /src/Core/Musify.Infrastructure \
     -e CONN="$CONN" \
     mcr.microsoft.com/dotnet/sdk:10.0 \
     sh -c 'dotnet tool install --global dotnet-ef >/dev/null 2>&1 || true