feat: stabilize KYC, international assets and multi-currency schema

- Split mother's name in KYC (last/first)
- Added mileage_unit and fuel_type to Assets
- Expanded AssetCost for international VAT and original currency
- Fixed SQLAlchemy IndexError in asset catalog lookup
- Added exchange_rate and ratings tables to models

backend/app/db/base_class.py

@@ -0,0 +1,13 @@
+from typing import Any
+from sqlalchemy.ext.declarative import as_declarative, declared_attr
+
+@as_declarative()
+class Base:
+    id: Any
+    __name__: str
+    
+    # Automatikusan generálja a tábla nevét az osztálynévből, 
+    # ha nincs külön megadva (bár mi megadjuk a sémát)
+    @declared_attr
+    def __tablename__(cls) -> str:
+        return cls.__name__.lower()