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_configurator/models/sale_order_line.py

@@ -641,3 +641,21 @@ class SaleOrderLine(models.Model):
                         'n': n,
                         'qty': int(line.product_uom_qty),
                     })
+
+
+    # ---- Customer references mirrored from parent sale.order ----------
+    # Related (not stored) — display-only on the line list so shipping /
+    # invoicing operators see the customer's job/PO ref per-line without
+    # navigating up to the order header.
+    x_fc_customer_job_number = fields.Char(
+        related='order_id.x_fc_customer_job_number',
+        string='Customer Job #',
+        readonly=True,
+        store=True,
+    )
+    x_fc_po_number = fields.Char(
+        related='order_id.x_fc_po_number',
+        string='Customer PO #',
+        readonly=True,
+        store=True,
+    )