feat(billing): _api_cancel_subscription service method + unit tests

Plan 01 (NexaCloud cutover) Task 1: cancel/close a subscription with the same
service-scoped authorization as _api_record_usage (resolve via
_fc_resolve_subscription; partner must be linked to this service). Idempotent
(no-op if already 6_churn). 5 unit tests, verified green on fcb_test
(fresh + l10n_ca). DELETE route + HttpCase follow in Task 2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_centralize_billing/tests/__init__.py

@@ -6,3 +6,4 @@ from . import test_webhook
 from . import test_importer
 from . import test_reconciliation
 from . import test_invoice_ledger
+from . import test_subscription_cancel