feat(billing): /usage resolves subscription by source app id (enables 2b)

_api_record_usage now resolves the target subscription via the source
app's own id (x_fc_nexacloud_subscription_id, scoped to the service)
before falling back to a direct Odoo sale.order id. This is what lets
NexaCloud push usage against the shadow subscriptions the importer
created from NexaCloud UUIDs — closing the flip-day mapping gap the
review flagged. Authz unchanged (partner must be linked to the service).

fusion_centralize_billing/tests/test_importer.py

@@ -254,3 +254,26 @@ class TestImporterReadGuard(TransactionCase):
         wiz = self.env['fusion.billing.import.wizard'].sudo().create({'dry_run': True})
         with self.assertRaises(UserError):
             wiz.action_test_connection()
+
+
+@tagged('post_install', '-at_install')
+class TestUsageApiSourceId(TransactionCase):
+    """The /usage API must resolve a subscription by NexaCloud's OWN id, so usage can be
+    pushed against shadow subs the importer created from UUIDs (the flip-day gap)."""
+
+    def setUp(self):
+        super().setUp()
+        self.env['fusion.billing.import.wizard'].sudo()._import_rows(_fixture())
+        self.service = self.env['fusion.billing.service'].search([('code', '=', 'nexacloud')])
+
+    def test_record_usage_resolves_by_nexacloud_subscription_id(self):
+        res = self.service._api_record_usage({'events': [{
+            'subscription_external_id': 's-1',   # NexaCloud UUID, not the Odoo id
+            'metric_code': 'cpu_seconds', 'quantity': 3600.0,
+            'period_start': '2026-05-01', 'period_end': '2026-06-01',
+            'idempotency_key': 'nc:s-1:2026-05'}]})
+        self.assertEqual(res['status'], 'ok')
+        self.assertEqual(res['accepted'], 1)
+        sub = self.env['sale.order'].search([('x_fc_nexacloud_subscription_id', '=', 's-1')])
+        usage = self.env['fusion.billing.usage'].search([('subscription_id', '=', sub.id)])
+        self.assertEqual(usage.quantity, 3600.0)