[FIX] product_sale_price_from_pricelist: migration timing fix

- Move migration from pre-migrate.py to post-migrate.py
- pre-migrate runs before ORM creates columns, causing 'column does not exist' errors
- post-migrate runs after columns are created, safe for updates
- Add check for column existence to handle fresh installations gracefully
- Ensures migration only runs when upgrading from older versions with data

product_sale_price_from_pricelist/migrations/18.0.2.1.0/post-migrate.py

@@ -21,6 +21,24 @@ def migrate(cr, version):
 
     _logger.info("Migrating price fields from product.template to product.product...")
 
+    # Check if columns exist before attempting migration
+    # (they may not exist in fresh installations)
+    cr.execute("""
+        SELECT EXISTS (
+            SELECT FROM information_schema.columns
+            WHERE table_name = 'product_product'
+            AND column_name = 'last_purchase_price_received'
+        );
+        """)
+    columns_exist = cr.fetchone()[0]
+
+    if not columns_exist:
+        _logger.info(
+            "Columns do not exist in product_product, skipping migration. "
+            "Fresh installation detected."
+        )
+        return
+
     # Migrate last_purchase_price_received
     cr.execute("""
         UPDATE product_product pp