Step 5: Update exchange price endpoint to use new pricing config

- Update ExchangeConfigResponse schema with direction-specific fields
- Remove premium_percentage from PriceResponse (now in config)
- Update price endpoint to load pricing config from database
- Update frontend to use direction-specific min/max and calculate premium
- Update tests to seed pricing config
- Add logic to clamp amount when direction changes

backend/schemas/price.py

@@ -17,21 +17,27 @@ class PriceHistoryResponse(BaseModel):
 class ExchangeConfigResponse(BaseModel):
     """Exchange configuration for the frontend."""
 
-    eur_min: int
-    eur_max: int
+    eur_min_buy: int
+    eur_max_buy: int
+    eur_min_sell: int
+    eur_max_sell: int
     eur_increment: int
-    premium_percentage: int
+    premium_buy: int
+    premium_sell: int
+    small_trade_threshold_eur: int
+    small_trade_extra_premium: int
 
 
 class PriceResponse(BaseModel):
     """Current BTC/EUR price for trading.
 
     Note: The actual agreed price depends on trade direction (buy/sell)
-    and is calculated by the frontend using market_price and premium_percentage.
+    and is calculated by the frontend using market_price and premium values.
+    Premium calculation: base premium for direction + extra premium if
+    trade <= threshold.
     """
 
     market_price: float  # Raw price from exchange
-    premium_percentage: int
     timestamp: datetime
     is_stale: bool