feat(billing): metered charge math (quota + overage)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fusion_centralize_billing/models/charge.py

@@ -1,7 +1,9 @@
 # -*- coding: utf-8 -*-
 # Copyright 2026 Nexa Systems Inc.
 # License OPL-1
-from odoo import fields, models
+import math
+
+from odoo import api, fields, models
 
 
 class FusionBillingCharge(models.Model):
@@ -51,3 +53,21 @@ class FusionBillingCharge(models.Model):
         "res.currency", default=lambda self: self.env.company.currency_id,
     )
     active = fields.Boolean(default=True)
+
+    def _compute_billable(self, total_quantity):
+        """Return (overage_units, amount) for total period usage under this charge.
+
+        - overage_units = usage above included_quota (never negative)
+        - 'standard'/'package'/'volume': priced per `unit_batch` block, partial block rounds up.
+          (graduated tiers are out of scope for the core; treated as 'standard'.)
+        """
+        self.ensure_one()
+        overage = max(0.0, (total_quantity or 0.0) - (self.included_quota or 0.0))
+        batch = self.unit_batch or 1.0
+        if self.charge_model == 'package':
+            # whole packages over the RAW quantity (quota ignored for package counting)
+            blocks = math.ceil((total_quantity or 0.0) / batch) if total_quantity else 0
+            return overage, round(blocks * (self.price_per_unit or 0.0), 2)
+        # standard / volume / graduated-fallback: price the overage in (rounded-up) batches
+        blocks = math.ceil(overage / batch) if overage > 0 else 0
+        return overage, round(blocks * (self.price_per_unit or 0.0), 2)