feat: internal code for automatic exchange rate fetching

app/apps/currencies/exchange_rates/base.py

@@ -0,0 +1,30 @@
+from abc import ABC, abstractmethod
+from decimal import Decimal
+from typing import List, Tuple, Optional
+from django.db.models import QuerySet
+
+from apps.currencies.models import Currency
+
+
+class ExchangeRateProvider(ABC):
+    rates_inverted = False
+
+    def __init__(self, api_key: Optional[str] = None):
+        self.api_key = api_key
+
+    @abstractmethod
+    def get_rates(
+        self, target_currencies: QuerySet, exchange_currencies: set
+    ) -> List[Tuple[Currency, Currency, Decimal]]:
+        """Fetch exchange rates for multiple currency pairs"""
+        raise NotImplementedError("Subclasses must implement get_rates method")
+
+    @classmethod
+    def requires_api_key(cls) -> bool:
+        """Return True if the service requires an API key"""
+        return True
+
+    @staticmethod
+    def invert_rate(rate: Decimal) -> Decimal:
+        """Invert the given rate."""
+        return Decimal("1") / rate