Adds POST /overlays/{id}/script, /wipe, /build under the overlay blueprint.
Generalizes /build to handle any owner/admin-editable overlay (deletes the
duplicate workshop-specific manual_build). Wipe runs the literal script
"find /overlay -mindepth 1 -delete" through run_sandboxed_script and
refuses with 409 while a build_overlay job is running. Adds an
admin-only system_wide=1 flag to POST /overlays for system-wide creation.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
