feat(certificates): multiple Default CoC Contacts per customer (M2o -> M2m)

res.partner.x_fc_default_coc_contact_id (single Many2one) becomes
x_fc_default_coc_contact_ids (self-referential Many2many 'Default CoC
Contacts') so a customer can list several contacts who need the CoC.

- res.partner: M2m field (rel fp_default_coc_contact_rel) + many2many_tags.
- Cert: contact_partner_id (primary addressee printed on the cert) is set to
  the FIRST CoC contact at job creation + lazy-filled at issue.
- Send: action_send_to_customer pre-fills the email composer with ALL the
  customer's CoC contacts (primary + the rest), falling back to the company.
- fp.job cert-default resolution + the action_issue gate wording updated.
- Migration 19.0.10.2.0: copies each partner's old single value into the new
  M2m, then drops the orphaned column.

Deployed + verified on entech: migration copied 2 existing values, old column
dropped, field is M2m, send pre-fills all contacts. entech-only part_line_ids
/ multi-part resolver 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_id
+          - contact_partner_id  ← partner.x_fc_default_coc_contact_ids[:1]
           - nc_quantity         ← qty_scrapped + qty_visual_insp_rejects
 
         Honours part.certificate_requirement (coc / coc_thickness /
@@ -2713,8 +2713,10 @@ class FpJob(models.Model):
             signer = spec.signer_user_id
         # Contact: per-customer default; blank means manager picks at issue.
         contact = False
-        if 'x_fc_default_coc_contact_id' in self.partner_id._fields:
-            contact = self.partner_id.x_fc_default_coc_contact_id
+        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]
         # NC qty: scrapped + visual rejects. Both NULL-safe.
         nc_qty = int(
             (self.qty_scrapped or 0)