feat(billing): usage-rating + webhook-dispatch crons

- SaleOrder._fc_rate_usage: aggregates usage, computes overage via
  charge._compute_billable, upserts sale.order.line for the overage product
- FusionBillingUsage._cron_rate_open_periods: hourly cron iterates active
  charges × in-progress subscriptions, calls _fc_rate_usage
- data/ir_cron.xml: two crons — rate usage (hourly), dispatch webhooks (2 min)
- __manifest__.py: registers data/ir_cron.xml in data list
- test_usage.py: test_rate_open_period_creates_overage_line (TDD, FCB_EXIT=0)

Reference: _create_recurring_invoice / _get_invoiceable_lines confirmed in
Enterprise sale_subscription/models/sale_order.py — overage line goes onto
sale.order so native invoicing picks it up via _get_invoiceable_lines.

fusion_centralize_billing/models/__init__.py

@@ -5,3 +5,4 @@ from . import charge
 from . import usage
 from . import webhook
 from . import reconciliation
+from . import sale_order