Step 1: Add PricingConfig model and PricingRepository

- Create PricingConfig model with all required fields (premium settings, trade limits)
- Implement PricingRepository with singleton pattern (get_current, create_or_update)
- Add comprehensive tests for repository functionality
- Export model and repository in __init__.py files

backend/repositories/__init__.py

@@ -4,6 +4,7 @@ from repositories.availability import AvailabilityRepository
 from repositories.exchange import ExchangeRepository
 from repositories.invite import InviteRepository
 from repositories.price import PriceRepository
+from repositories.pricing import PricingRepository
 from repositories.role import RoleRepository
 from repositories.user import UserRepository
 
@@ -12,6 +13,7 @@ __all__ = [
     "ExchangeRepository",
     "InviteRepository",
     "PriceRepository",
+    "PricingRepository",
     "RoleRepository",
     "UserRepository",
 ]