feat(billing): 2a NexaCloud→Odoo importer (read-only, idempotent, shadow-safe)

fusion.billing.import.wizard backfills NexaCloud into Odoo: read-only
psycopg2 reader (_read_nexacloud_rows, DSN from ir.config_parameter)
split from pure-Odoo writes (_import_rows/_do_import) so the logic is
unit-tested headless. Maps users→partners+links (reusing
_resolve_or_create_partner, stashing stripe_customer_id), plans→a
cpu_seconds charge catalog (included_quota=cpu_seconds_quota,
unit_batch=3600, $0.0075/core-hour, plan_id NULL), and deployments→one
DRAFT shadow sale.order per deployment with the flat price set
explicitly. Shadow-safe by construction: draft + no payment token +
charge plan_id NULL (rating cron is a no-op). Idempotent re-runs;
per-row savepoints isolate bad rows; dry-run rolls back. 11 tests,
50/50 green on odoo-trial.

fusion_centralize_billing/models/__init__.py

@@ -6,3 +6,4 @@ from . import usage
 from . import webhook
 from . import reconciliation
 from . import sale_order
+from . import res_partner