feat(certificates): cert Customer Contact is multi-contact, auto-filled, sends to all

fp.certificate.contact_partner_id (single 'Customer Contact') becomes
contact_partner_ids (Many2many) — same shape as the partner's Default CoC
Contacts, as requested.

- Auto-populate: at job creation (fp.job cert resolution) + lazy-fill at issue,
  contact_partner_ids = the customer's x_fc_default_coc_contact_ids (ALL).
- Send: action_send_to_customer pre-fills the composer with exactly the cert's
  contact_partner_ids, so the CoC goes to all the defined clients (fallback:
  company).
- Primary: the FIRST contact prints on the CoC + is gated for email; report
  uses contact_partner_ids[:1].
- Gate: requires >=1 Customer Contact + the primary has an email.
- View: many2many_tags.
- Migration 19.0.10.3.0: copies each cert's old single contact into the new M2m,
  drops the orphaned column.

Deployed + verified on entech: migration copied 16 certs, old column dropped,
field is M2m, send pre-fills the cert contacts, CoC report renders. entech-only
part_line_ids preserved.

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

fusion_plating/fusion_plating_jobs/models/fp_job.py

@@ -2678,7 +2678,7 @@ class FpJob(models.Model):
                                   (a per-spec override). Left empty
                                   otherwise so the LIVE company owner
                                   resolves at render / issue time.
-          - contact_partner_id  ← partner.x_fc_default_coc_contact_ids[:1]
+          - contact_partner_ids ← partner.x_fc_default_coc_contact_ids (all)
           - nc_quantity         ← qty_scrapped + qty_visual_insp_rejects
 
         Honours part.certificate_requirement (coc / coc_thickness /
@@ -2714,9 +2714,10 @@ class FpJob(models.Model):
         # Contact: per-customer default; blank means manager picks at issue.
         contact = False
         if 'x_fc_default_coc_contact_ids' in self.partner_id._fields:
-            # Primary = first of the customer's CoC contacts; the rest are
-            # CC'd at send (fp.certificate.action_send_to_customer).
-            contact = self.partner_id.x_fc_default_coc_contact_ids[:1]
+            # ALL the customer's CoC contacts -> the cert's Customer Contact
+            # M2m. First is the primary (printed on the CoC); every contact
+            # is emailed at send (fp.certificate.action_send_to_customer).
+            contact = self.partner_id.x_fc_default_coc_contact_ids
         # NC qty: scrapped + visual rejects. Both NULL-safe.
         nc_qty = int(
             (self.qty_scrapped or 0)
@@ -2786,8 +2787,8 @@ class FpJob(models.Model):
                     vals['process_description'] = recipe.name or ''
                 if 'certified_by_id' in Cert._fields and signer:
                     vals['certified_by_id'] = signer.id
-                if 'contact_partner_id' in Cert._fields and contact:
-                    vals['contact_partner_id'] = contact.id
+                if 'contact_partner_ids' in Cert._fields and contact:
+                    vals['contact_partner_ids'] = [(6, 0, contact.ids)]
                 if 'entech_wo_number' in Cert._fields:
                     vals['entech_wo_number'] = self.name or ''
                 cert = Cert.create(vals)