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_certificates/views/fp_certificate_views.xml

@@ -101,7 +101,8 @@
                         <group>
                             <field name="certificate_type"/>
                             <field name="partner_id"/>
-                            <field name="contact_partner_id"
+                            <field name="contact_partner_ids"
+                                   widget="many2many_tags"
                                    options="{'no_create': True}"
                                    invisible="not partner_id"/>
                             <field name="sale_order_id"/>