fix: Remove agreed_price from price API response

The agreed_price depends on trade direction (buy/sell) and must be
calculated on the frontend. Returning a buy-side-only agreed_price
from the API was misleading and unused.

Frontend already calculates the direction-aware price correctly.

backend/routes/exchange.py

@@ -61,10 +61,13 @@ class ExchangeConfigResponse(BaseModel):
 
 
 class PriceResponse(BaseModel):
-    """Current BTC/EUR price with premium applied."""
+    """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.
+    """
 
     market_price: float  # Raw price from exchange
-    agreed_price: float  # Price with premium applied
     premium_percentage: int
     timestamp: datetime
     is_stale: bool
@@ -115,13 +118,6 @@ def apply_premium_for_direction(
         return market_price * (1 - premium_percentage / 100)
 
 
-def apply_premium(market_price: float, premium_percentage: int) -> float:
-    """Apply buy-side premium (for price display)."""
-    return apply_premium_for_direction(
-        market_price, premium_percentage, TradeDirection.BUY
-    )
-
-
 def calculate_sats_amount(
     eur_cents: int,
     price_eur_per_btc: float,
@@ -204,7 +200,7 @@ async def get_exchange_price(
 
     The response includes:
     - market_price: The raw price from the exchange
-    - agreed_price: The price with admin premium applied
+    - premium_percentage: The premium to apply to trades
     - is_stale: Whether the price is older than 5 minutes
     - config: Trading configuration (min/max EUR, increment)
     """
@@ -237,7 +233,6 @@ async def get_exchange_price(
             return ExchangePriceResponse(
                 price=PriceResponse(
                     market_price=price_value,
-                    agreed_price=apply_premium(price_value, PREMIUM_PERCENTAGE),
                     premium_percentage=PREMIUM_PERCENTAGE,
                     timestamp=timestamp,
                     is_stale=False,
@@ -250,9 +245,6 @@ async def get_exchange_price(
                 return ExchangePriceResponse(
                     price=PriceResponse(
                         market_price=cached_price.price,
-                        agreed_price=apply_premium(
-                            cached_price.price, PREMIUM_PERCENTAGE
-                        ),
                         premium_percentage=PREMIUM_PERCENTAGE,
                         timestamp=cached_price.timestamp,
                         is_stale=True,
@@ -271,7 +263,6 @@ async def get_exchange_price(
     return ExchangePriceResponse(
         price=PriceResponse(
             market_price=cached_price.price,
-            agreed_price=apply_premium(cached_price.price, PREMIUM_PERCENTAGE),
             premium_percentage=PREMIUM_PERCENTAGE,
             timestamp=cached_price.timestamp,
             is_stale=is_price_stale(cached_price.timestamp),