/fp/workspace/sign_off: signature_data_uri now optional; a supplied drawing
persists to res.users.x_fc_signature_image (SELF_WRITEABLE) and the wasted
per-step ir.attachment is dropped; no drawing + a saved signature just finishes.
/fp/workspace/load exposes user_has_plating_signature + user_plating_signature.
Merged 3 new tests into the existing TestWorkspaceSignOff.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>