[IMP] product_pricelist_total_margin: Add margin_type selector (Markup vs Commercial Margin)

- Add margin_type field to choose between calculation methods:
  * Markup (on cost): PVP = Cost × (1 + markup%)
  * Commercial Margin (on PVP): PVP = Cost / (1 - margin%)
- Update _compute_price() to apply correct formula based on margin_type
- Add safety cap for commercial margin >= 100% (caps at 99%)
- Add detailed logging for both calculation types
- Update views to show margin_type field when use_total_margin is enabled
- Add 2 new tests to validate both calculation methods:
  * test_total_margin_markup_type: validates markup formula
  * test_total_margin_commercial_margin_type: validates commercial margin formula
- All 11 tests passing (9 existing + 2 new)

Example with Commercial Margin:
Base: 4.68€, Total Margin: 20%
- Markup: 4.68 × 1.20 = 5.616€ (margin = 16.67% of PVP)
- Commercial Margin: 4.68 / 0.80 = 5.85€ (margin = 20% of PVP) ✓

product_pricelist_total_margin/models/product_pricelist_item.py

@@ -23,6 +23,20 @@ class ProductPricelistItem(models.Model):
         "- Total (this option): 100 * (1 + 0.20) = 120€",
     )
 
+    margin_type = fields.Selection(
+        selection=[
+            ("markup", "Markup (on cost)"),
+            ("margin", "Commercial Margin (on PVP)"),
+        ],
+        string="Margin Type",
+        default="markup",
+        help="Type of margin calculation:\n"
+        "- Markup: PVP = Cost × (1 + markup%). Margin is calculated on cost.\n"
+        "  Example: Cost 100€, Markup 25% → PVP = 125€\n\n"
+        "- Commercial Margin: PVP = Cost / (1 - margin%). Margin is calculated on PVP.\n"
+        "  Example: Cost 100€, Margin 20% → PVP = 125€ (margin = 25€/125€ = 20%)",
+    )
+
     def _get_base_price_and_margins(self, product, quantity, uom, date, currency):
         """
         Traverse the pricelist chain to get the original base price and collect
@@ -367,14 +381,32 @@ class ProductPricelistItem(models.Model):
             # Apply global min/max margin limits
             total_margin = self._apply_global_margin_limits(total_margin, base_price)
 
-            # Apply total margin to base price
-            price = base_price * (1 + total_margin / 100)
-            _logger.info(
-                "[TOTAL MARGIN] Base price %.2f * (1 + %.2f%%) = %.2f",
-                base_price,
-                total_margin,
-                price,
-            )
+            # Apply total margin to base price using selected margin type
+            margin_type = self.margin_type or "markup"
+            if margin_type == "margin":
+                # Commercial Margin: PVP = Cost / (1 - margin%)
+                if total_margin >= 100:
+                    _logger.warning(
+                        "[TOTAL MARGIN] Commercial margin %.2f%% >= 100%%, capping at 99%%",
+                        total_margin,
+                    )
+                    total_margin = 99.0
+                price = base_price / (1 - total_margin / 100)
+                _logger.info(
+                    "[TOTAL MARGIN] Commercial Margin: Base %.2f / (1 - %.2f%%) = %.2f",
+                    base_price,
+                    total_margin,
+                    price,
+                )
+            else:
+                # Markup: PVP = Cost × (1 + markup%)
+                price = base_price * (1 + total_margin / 100)
+                _logger.info(
+                    "[TOTAL MARGIN] Markup: Base %.2f * (1 + %.2f%%) = %.2f",
+                    base_price,
+                    total_margin,
+                    price,
+                )
 
             # Apply formula extras (round, surcharge, min/max margins)
             price = self._apply_formula_extras(price, base_price, currency)