feat(fusion_plating): Express masking reference images → mask step + workstation viewer

Order-entry shortcut: when masking is toggled ON for an Express order line,
an amber "MASK" button appears to attach reference image(s)/PDF(s). The files
ride the existing _fp_apply_express_overrides_to_job path onto the job's
masking step, so the operator sees exactly what to mask — no recipe edit or
custom prompt needed.

- configurator: masking_attachment_ids on the wizard line + SO line;
  action_upload_masking_ref; override branch writes refs onto mask steps;
  amber multi-file MASK button (express_action_btns) shown when masking is on.
- jobs: x_fc_masking_attachment_ids on fp.job.step (per-step) + computed
  rollup on fp.job; office "Masking Refs" form page (readonly preview).
- shopfloor: workspace step payload carries masking_refs (sudo'd attachment
  read, rule 13m); operator sees thumbnail/PDF tiles on the mask step that
  open in Odoo's full-screen FileViewer (zoom + swipe).

Verified end-to-end on entech: SO-line refs land on the mask step + job
rollup (WO-30091); payload mask_refs shape correct (is_image, /web/image).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_configurator/views/fp_express_order_views.xml

@@ -279,6 +279,7 @@
                             <field name="customer_line_ref" string="Line Job #" placeholder="ABC" width="80px"/>
                             <field name="thickness_range" string="Thickness" placeholder=".0005-.0010" width="100px"/>
                             <field name="masking_enabled" string="Mask" widget="boolean_toggle" width="55px"/>
+                            <field name="masking_attachment_ids" column_invisible="1"/>
                             <!-- Bake pill — click to edit -->
                             <field name="bake_instructions"
                                    string="Bake"