Phase 1.1: Add exchange configuration

- Add exchange constants to shared/constants.json:
  - eurTradeMin: 100, eurTradeMax: 3000, eurTradeIncrement: 20
  - premiumPercentage: 5
  - priceRefreshSeconds: 60, priceStalenessSeconds: 300
- Add exchangeStatuses and tradeDirections to shared constants
- Add ExchangeStatus and TradeDirection enums to models.py
- Update shared_constants.py to export new exchange constants
- Update validate_constants.py to validate new enums and fields

backend/models.py

@@ -68,6 +68,23 @@ class AppointmentStatus(str, PyEnum):
     CANCELLED_BY_ADMIN = "cancelled_by_admin"
 
 
+class ExchangeStatus(str, PyEnum):
+    """Status of an exchange trade."""
+
+    BOOKED = "booked"
+    COMPLETED = "completed"
+    CANCELLED_BY_USER = "cancelled_by_user"
+    CANCELLED_BY_ADMIN = "cancelled_by_admin"
+    NO_SHOW = "no_show"
+
+
+class TradeDirection(str, PyEnum):
+    """Direction of a trade from the user's perspective."""
+
+    BUY = "buy"  # User buys BTC, gives EUR
+    SELL = "sell"  # User sells BTC, gets EUR
+
+
 # Role name constants
 ROLE_ADMIN = "admin"
 ROLE_REGULAR = "regular"