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/models/__init__.py

@@ -14,6 +14,7 @@ from .enums import (
 from .exchange import Exchange
 from .invite import Invite
 from .price_history import PriceHistory
+from .pricing_config import PricingConfig
 
 # Export role configuration
 from .role_config import ROLE_ADMIN, ROLE_DEFINITIONS, ROLE_REGULAR
@@ -34,6 +35,7 @@ __all__ = [
     "InviteStatus",
     "Permission",
     "PriceHistory",
+    "PricingConfig",
     "Role",
     "RoleConfig",
     "TradeDirection",