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/models/connections.py

@@ -59,7 +59,6 @@ class SyncConfig(Base):
     is_enabled = Column(Boolean, nullable=False, default=False)
     confirmed_at = Column(DateTime, nullable=True)
     price_multiplier = Column(Numeric(10, 4), nullable=False, default=1.0)
-    description_postfix = Column(String(255), nullable=True)
     created_at = Column(DateTime, nullable=False, server_default=func.now())
     updated_at = Column(DateTime, nullable=False, server_default=func.now(), onupdate=func.now())