fix(billing): resolve code-review findings (authz, cross-billing, validation, webhook integrity)

- C1/H4: rating cron only rates subs on the charge's own plan_id
- C1: _fc_rate_usage skips creating a line when amount is 0 (still updates existing)
- C2/C4: /usage authorizes each event (exists + is_subscription + linked customer)
- C3: API handlers validate input and return 4xx-shaped errors instead of raising;
       controller maps status=='error' to HTTP 400
- H1: cron uses real billing window [last_invoice_date or start_date, next_invoice_date)
- H2: _aggregate uses half-open window anchored on period_start
- H3: idempotency scoped to (subscription_id, metric_id, idempotency_key)
- H5: webhook stores canonical body, signs+POSTs it verbatim, adds X-Fusion-Event-Id,
       caps backoff at 2**min(attempts,10)
- H6: SSRF guard rejects non-https / localhost / private / link-local webhook_url
- M7: charge_model reduced to standard/package (dropped unimplemented graduated/volume)
- L1: currency_id required on charge + reconciliation
- L2: charge price non-negative + unit_batch positive DB constraints

Adds 17 regression tests (suite 22 -> 39, all green via fcb_test_on_trial.sh).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_centralize_billing/models/sale_order.py

@@ -10,13 +10,19 @@ class SaleOrder(models.Model):
     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."""
+        Returns the amount.
+
+        A zero amount never *creates* a new line (no $0.00 overage clutter); if a
+        line already exists it is still updated so a dropped-to-zero overage clears.
+        """
         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)
+            if not line and amount == 0:
+                return amount
             vals = {'product_uom_qty': 1, 'price_unit': amount}
             if line:
                 line.write(vals)