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/__manifest__.py

@@ -47,6 +47,7 @@ reference files from the container before implementing subscription/account inte
     ],
     "data": [
         "security/ir.model.access.csv",
+        "data/ir_cron.xml",
     ],
     "installable": True,
     "application": False,