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

shared/constants.json

@@ -13,12 +13,31 @@
     "CANCELLED_BY_USER": "cancelled_by_user",
     "CANCELLED_BY_ADMIN": "cancelled_by_admin"
   },
+  "exchangeStatuses": {
+    "BOOKED": "booked",
+    "COMPLETED": "completed",
+    "CANCELLED_BY_USER": "cancelled_by_user",
+    "CANCELLED_BY_ADMIN": "cancelled_by_admin",
+    "NO_SHOW": "no_show"
+  },
+  "tradeDirections": {
+    "BUY": "buy",
+    "SELL": "sell"
+  },
   "booking": {
     "slotDurationMinutes": 15,
     "maxAdvanceDays": 30,
     "minAdvanceDays": 1,
     "noteMaxLength": 144
   },
+  "exchange": {
+    "eurTradeMin": 100,
+    "eurTradeMax": 3000,
+    "eurTradeIncrement": 20,
+    "premiumPercentage": 5,
+    "priceRefreshSeconds": 60,
+    "priceStalenessSeconds": 300
+  },
   "validation": {
     "telegram": {
       "maxLengthAfterAt": 32,