feat(billing): NexaCloud invoice ledger — ingest invoices to account.move

Odoo becomes the accounting SoR by ingesting NexaCloud's real Stripe
invoices (read-only via the existing DSN) into native account.move
customer invoices: per-service-family income accounts, tax derived to
match the source invoice.tax, Stripe payments reconciled via
account.payment.register (invoice shows paid), idempotent on
x_fc_nexacloud_invoice_id, draft-first with bulk-post + a daily cron
(inactive). Plus a prune helper for the now-obsolete metered shadow data.
73 tests green on odoo-trial. Account codes use dots (Odoo 19 rejects '-').

fusion_centralize_billing/models/__init__.py

@@ -7,3 +7,4 @@ from . import webhook
 from . import reconciliation
 from . import sale_order
 from . import res_partner
+from . import account_move