refactor: derive VK description postfix from measure_name, drop global postfix setting

Each product's description is now built as "Name (цена за M.)" using its
own measure_name. The global description_postfix setting is removed —
it couldn't handle per-product units.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/tasks/vk_sync.py

@@ -153,8 +153,8 @@ def _sync_product(
     name = _name_for_vk(product.name)
     multiplier = float(sync_config.price_multiplier) if sync_config and sync_config.price_multiplier else 1.0
     price_rubles = _calc_price(product.price) * multiplier
-    postfix = (sync_config.description_postfix or "").strip() if sync_config else ""
-    desc = f"{product.name} ({postfix})" if postfix else product.name
+    measure = (product.measure_name or "").strip()
+    desc = f"{product.name} (цена за {measure}.)" if measure else product.name
     stock = settings.VK_STOCK_AMOUNT if product.allow_to_sell else 0
     owner_id = f"-{vk_group_id}"
     now = _now()