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_certificates/models/res_partner.py

@@ -128,17 +128,24 @@ class ResPartner(models.Model):
              'who require specific NIST or DFARS language.',
     )
 
-    # ---- Default CoC contact (cert addressee + email recipient) ----------
-    # The single named contact printed on the CoC and used as the email
-    # default when the cert ships. Sales sets it once per customer.
-    # Falls back to manual selection at action_issue time if blank.
-    x_fc_default_coc_contact_id = fields.Many2one(
+    # ---- Default CoC contacts (cert addressees + email recipients) -------
+    # One or more named contacts who receive this customer's CoC. The first
+    # is the primary (printed on the cert + pre-fills cert.contact_partner_id
+    # when a job ships); the rest are CC'd when the CoC is emailed. Sales
+    # sets the list once per customer. Falls back to manual selection at
+    # action_issue time if blank. Self-referential M2m (renamed from the old
+    # single Many2one x_fc_default_coc_contact_id — see migration 19.0.10.2.0).
+    x_fc_default_coc_contact_ids = fields.Many2many(
         'res.partner',
-        string='Default CoC Contact',
+        relation='fp_default_coc_contact_rel',
+        column1='partner_id', column2='contact_id',
+        string='Default CoC Contacts',
         domain="[('parent_id', '=', id), ('is_company', '=', False)]",
         tracking=True,
-        help='Default contact the Certificate of Conformance is addressed '
-             'to and emailed to. Pre-fills cert.contact_partner_id when a '
-             'job ships. Leave blank to force the manager to pick at '
-             'issue time. Must be a child contact of this company.',
+        help='Contacts the Certificate of Conformance is addressed to and '
+             'emailed to. The first contact is the primary (printed on the '
+             'cert and pre-filled as the cert contact when a job ships); '
+             'the rest are copied (CC) when the CoC is sent. Leave blank to '
+             'force the manager to pick at issue time. Child contacts of '
+             'this company only.',
     )