feat(billing): identity resolution external account -> partner

fusion_centralize_billing/models/account_link.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2026 Nexa Systems Inc.
 # License OPL-1
-from odoo import fields, models
+from odoo import api, fields, models
 
 
 class FusionBillingAccountLink(models.Model):
@@ -31,3 +31,27 @@ class FusionBillingAccountLink(models.Model):
         "unique(service_id, external_id)",
         "An external account can only link to one partner per service.",
     )
+
+    @api.model
+    def _resolve_or_create_partner(self, service, external_id, name=None, email=None, extra=None):
+        """Return the link for (service, external_id), creating partner+link if needed.
+
+        Unifies customers: if a link for this external_id exists, reuse it; else if a
+        partner with the same email already exists (possibly from another service),
+        link to it; else create a new partner.
+        """
+        existing = self.search(
+            [('service_id', '=', service.id), ('external_id', '=', external_id)], limit=1)
+        if existing:
+            return existing
+        partner = self.env['res.partner']
+        if email:
+            partner = partner.search([('email', '=', email)], limit=1)
+        if not partner:
+            partner = partner.create({'name': name or external_id, 'email': email, **(extra or {})})
+        return self.create({
+            'service_id': service.id,
+            'external_id': external_id,
+            'external_email': email,
+            'partner_id': partner.id,
+        })