fix(billing): charge rate precision — Float not Monetary, no premature cent-rounding

price_per_unit was a Monetary field, so a realistic sub-cent rate like
$0.0075/core-hour was rounded to $0.01 on write, corrupting the rate.
Make it Float(16,6). Also stop _compute_billable from rounding the
overage amount to 2 decimals mid-calc — that lost the half-cent on
sub-cent rates and would drift against the source app, which keeps
usage amounts at 4 decimals and only rounds at the invoice total.
Now rounds to 6 dp (float-noise only); cent-rounding defers to the
invoice line. Exposed while building the NexaCloud importer.

fusion_centralize_billing/models/charge.py

@@ -36,7 +36,13 @@ class FusionBillingCharge(models.Model):
     included_quota = fields.Float(
         default=0.0, help="Units included before overage applies, per period.",
     )
-    price_per_unit = fields.Monetary(help="Overage price per unit_batch.")
+    price_per_unit = fields.Float(
+        digits=(16, 6),
+        help="Overage price per unit_batch. A Float (not Monetary) so sub-cent rates "
+             "like $0.0075/core-hour are stored exactly — Monetary rounds to the "
+             "currency's 2 decimals and would corrupt the rate. Final cent-rounding "
+             "happens at the invoice line/total, not in the per-charge math.",
+    )
     unit_batch = fields.Float(
         default=1.0, help="Batch size for overage pricing, e.g. 1000 = priced per 1k.",
     )
@@ -67,6 +73,12 @@ class FusionBillingCharge(models.Model):
         - 'standard': price the overage in (rounded-up) `unit_batch` blocks.
         - 'package': price whole packages over the RAW quantity (quota ignored for
           package counting); a partial package rounds up.
+
+        The amount keeps the rate's precision (rounded to 6 dp only to clear float
+        noise) — it must NOT be rounded to cents here. Sub-cent rates (e.g.
+        $0.0075/core-hour) and fractional totals are preserved so they match the
+        source app's own sub-cent usage amounts; final cent-rounding happens once at
+        the invoice line / invoice total, exactly as the source app does.
         """
         self.ensure_one()
         overage = max(0.0, (total_quantity or 0.0) - (self.included_quota or 0.0))
@@ -74,7 +86,7 @@ class FusionBillingCharge(models.Model):
         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)
+            return overage, round(blocks * (self.price_per_unit or 0.0), 6)
         # standard: 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)
+        return overage, round(blocks * (self.price_per_unit or 0.0), 6)