Shop-floor sign-off currently makes operators redraw a signature every
time, and the drawing is discarded (reports read x_fc_signature_image).
Spec: use the saved Plating Signature (one-tap confirm-with-preview);
draw once when absent and persist it to x_fc_signature_image so future
sign-offs + reports reuse it. Tablet-workspace scope; no model/migration.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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