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/data/ir_cron.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo noupdate="1">
+    <record id="cron_fc_rate_usage" model="ir.cron">
+        <field name="name">Fusion Billing: Rate usage before invoicing</field>
+        <field name="model_id" ref="model_fusion_billing_usage"/>
+        <field name="state">code</field>
+        <field name="code">model._cron_rate_open_periods()</field>
+        <field name="interval_number">1</field>
+        <field name="interval_type">hours</field>
+        <field name="active">True</field>
+    </record>
+
+    <record id="cron_fc_dispatch_webhooks" model="ir.cron">
+        <field name="name">Fusion Billing: Dispatch outbound webhooks</field>
+        <field name="model_id" ref="model_fusion_billing_webhook"/>
+        <field name="state">code</field>
+        <field name="code">model._cron_dispatch()</field>
+        <field name="interval_number">2</field>
+        <field name="interval_type">minutes</field>
+        <field name="active">True</field>
+    </record>
+</odoo>