#51 fix: detectar cambios de categorias al sincronizar productos

Issue #51: discriminacion y administracion de categorias de productos.

- _need_update compara las categorias (external_id) local vs Tryton
- _describe_changes reporta categorias agregadas (+Nombre) y removidas (-Nombre)
- Actualiza test que asumia asignacion de categorias sin marcar update

tienda_ilusion/don_confiao/services/tryton/products.py

@@ -214,8 +214,50 @@ class ProductTrytonService:
                 changes.append(
                     f"Unidad: {product.measuring_unit} → {unit_name}"
                 )
+        category_changes = self._describe_category_changes(
+            product, tryton_product
+        )
+        if category_changes:
+            changes.append(category_changes)
         return ", ".join(changes)
 
+    def _describe_category_changes(self, product, tryton_product):
+        """Describe los cambios de categorías entre el producto local y Tryton"""
+        tryton_ids = set(
+            str(category_id)
+            for category_id in self._extract_category_ids(tryton_product)
+        )
+        local = {
+            external_id: name
+            for external_id, name in product.categories.values_list(
+                "external_id", "name"
+            )
+            if external_id
+        }
+        local_ids = set(local)
+
+        added = sorted(tryton_ids - local_ids)
+        removed = sorted(local_ids - tryton_ids)
+        if not added and not removed:
+            return ""
+
+        parts = []
+        if added:
+            names = {}
+            for external_id in added:
+                category = (
+                    ProductCategory.objects.filter(
+                        external_id=external_id
+                    )
+                    .values_list("name", flat=True)
+                    .first()
+                )
+                names[external_id] = category or external_id
+            parts.append(", ".join(f"+{names[e]}" for e in added))
+        if removed:
+            parts.append(", ".join(f"-{local[e]}" for e in removed))
+        return f"Categoría: {', '.join(parts)}"
+
     @staticmethod
     def _format_price(value):
         """Formatea un precio sin ceros decimales innecesarios"""
@@ -269,6 +311,17 @@ class ProductTrytonService:
         unit = tryton_product.get("default_uom.")
         if not product.measuring_unit == unit.get("rec_name"):
             return True
+        tryton_category_ids = set(
+            str(category_id)
+            for category_id in self._extract_category_ids(tryton_product)
+        )
+        local_category_ids = set(
+            product.categories.exclude(
+                external_id__isnull=True
+            ).values_list("external_id", flat=True)
+        )
+        if tryton_category_ids != local_category_ids:
+            return True
         return False
 
     def _create_product(self, tryton_product):