feat(jobs): persist Customer Job # + PO # + Rush Order on fp.job

Tier 1 of the SO->fp.job persistence audit. Three customer-reference
fields entered on sale.order's Plating tab were not flowing through
to fp.job (or SO lines), so the shop floor and printed paperwork
(traveller, BoL, cert) had to round-trip via sale_order_id every time.

Changes:
- fp.job: new x_fc_customer_job_number (Char, tracking), x_fc_po_number
  (Char, tracking), x_fc_rush_order (Boolean, tracking). All three
  populated by _fp_auto_create_job at SO confirm time.
- sale.order.line: x_fc_customer_job_number / x_fc_po_number added as
  stored related fields off order_id so per-line list views show the
  customer's references without navigating to the order header
  (x_fc_rush_order was already on lines).
- fp.job form view: small Customer References group under the title
  surfaces the three fields where the user expects them.

Verified end-to-end: SO -> SO line related fields -> fp.job direct
fields all carry the same value.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_jobs/models/sale_order.py

@@ -322,6 +322,16 @@ class SaleOrder(models.Model):
             if recipe:
                 vals['recipe_id'] = recipe.id
 
+            # Customer references — mirror onto the job so the shop floor
+            # has them without round-tripping to the SO.
+            if 'x_fc_customer_job_number' in self._fields \
+                    and self.x_fc_customer_job_number:
+                vals['x_fc_customer_job_number'] = self.x_fc_customer_job_number
+            if 'x_fc_po_number' in self._fields and self.x_fc_po_number:
+                vals['x_fc_po_number'] = self.x_fc_po_number
+            if 'x_fc_rush_order' in self._fields:
+                vals['x_fc_rush_order'] = bool(self.x_fc_rush_order)
+
             # Customer spec / facility / manager — copy from SO if present
             if 'x_fc_customer_spec_id' in self._fields and self.x_fc_customer_spec_id:
                 vals['customer_spec_id'] = self.x_fc_customer_spec_id.id