When deleting a hosted site, the associated Store is now soft-deleted
(sets deleted_at). This frees the subdomain for reuse via the partial
unique index (WHERE deleted_at IS NULL).
Previously the Store was orphaned, blocking subdomain reuse.
Closes docs/proposals/hosting-cascade-delete.md.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>