feat(billing): period usage aggregation by metric function

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

fusion_centralize_billing/models/usage.py

@@ -58,3 +58,27 @@ class FusionBillingUsage(models.Model):
                 existing.write({'quantity': quantity})
                 return existing
         return self.create(vals)
+
+    @api.model
+    def _aggregate(self, subscription, metric, period_start, period_end):
+        """Aggregate stored usage for a subscription+metric within [period_start, period_end)
+        using the metric's aggregation function."""
+        rows = self.search([
+            ('subscription_id', '=', subscription.id),
+            ('metric_id', '=', metric.id),
+            ('period_start', '>=', period_start),
+            ('period_end', '<=', period_end),
+        ])
+        qtys = rows.mapped('quantity')
+        if not qtys:
+            return 0.0
+        agg = metric.aggregation
+        if agg == 'sum':
+            return sum(qtys)
+        if agg == 'max':
+            return max(qtys)
+        if agg == 'last':
+            return rows.sorted('period_start')[-1].quantity
+        if agg == 'unique_count':
+            return float(len(set(qtys)))
+        return sum(qtys)