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

fusion_centralize_billing/models/sale_order.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1
+from odoo import api, fields, models
+
+
+class SaleOrder(models.Model):
+    _inherit = "sale.order"
+
+    def _fc_rate_usage(self, charge, period_start, period_end):
+        """Aggregate this subscription's usage for `charge`'s metric in the period,
+        compute the overage amount, and upsert a matching overage order line.
+        Returns the amount."""
+        self.ensure_one()
+        Usage = self.env['fusion.billing.usage']
+        total = Usage._aggregate(self, charge.metric_id, period_start, period_end)
+        _overage, amount = charge._compute_billable(total)
+        if charge.product_id:
+            line = self.order_line.filtered(lambda l: l.product_id == charge.product_id)
+            vals = {'product_uom_qty': 1, 'price_unit': amount}
+            if line:
+                line.write(vals)
+            else:
+                self.env['sale.order.line'].create(
+                    {'order_id': self.id, 'product_id': charge.product_id.id, **vals})
+        return amount

fusion_centralize_billing/models/usage.py

@@ -59,6 +59,25 @@ class FusionBillingUsage(models.Model):
                 return existing
         return self.create(vals)
 
+    @api.model
+    def _cron_rate_open_periods(self):
+        """Hourly cron: for every active charge, aggregate usage and upsert overage lines
+        on all in-progress subscriptions whose next invoice date is set."""
+        Charge = self.env['fusion.billing.charge'].search([('active', '=', True)])
+        SaleOrder = self.env['sale.order']
+        for charge in Charge:
+            subs = SaleOrder.search([
+                ('is_subscription', '=', True),
+                ('subscription_state', '=', '3_progress'),
+                ('plan_id.name', '!=', False),
+            ])
+            for sub in subs:
+                if not sub.next_invoice_date:
+                    continue
+                period_end = fields.Datetime.to_datetime(sub.next_invoice_date)
+                period_start = period_end.replace(day=1)
+                sub._fc_rate_usage(charge, period_start, period_end)
+
     @api.model
     def _aggregate(self, subscription, metric, period_start, period_end):
         """Aggregate stored usage for a subscription+metric within [period_start, period_end)