feat(billing): 2d dual-run reconciliation (Odoo-computed vs NexaCloud-actual)

fusion.billing.reconciliation gains the compute: _compute_reconciliation
(flat + charge overage vs external, status match/delta at a tolerance) and
_reconcile_rows (resolve shadow sub -> flat + charge, upsert one row per
service/partner/period, per-row isolated). The wizard gains a read-only
_read_reconciliation_rows (NexaCloud usage cpu_hours*3600 + invoice-item
subtotals per YYYY-MM) and a "Run Reconciliation" button. 2a amended to
stamp x_fc_nexacloud_plan_id on shadow subs so reconciliation can find the
charge. Read-only on NexaCloud; writes only reconciliation rows (shadow
guarantees intact). 8 new tests, full suite green on odoo-trial.

fusion_centralize_billing/tests/__init__.py

@@ -4,3 +4,4 @@ from . import test_usage
 from . import test_api
 from . import test_webhook
 from . import test_importer
+from . import test_reconciliation