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/main.py

@@ -1,11 +1,15 @@
 """FastAPI application entry point."""
 
+import asyncio
+import logging
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from database import Base, engine
+from database import Base, async_session, engine
+from models import PriceHistory
+from price_fetcher import PAIR_BTC_EUR, SOURCE_BITFINEX, fetch_btc_eur_price
 from routes import audit as audit_routes
 from routes import auth as auth_routes
 from routes import availability as availability_routes
@@ -13,19 +17,63 @@ from routes import exchange as exchange_routes
 from routes import invites as invites_routes
 from routes import meta as meta_routes
 from routes import profile as profile_routes
+from shared_constants import PRICE_REFRESH_SECONDS
 from validate_constants import validate_shared_constants
 
+logger = logging.getLogger(__name__)
+
+# Background task handle
+_price_fetch_task: asyncio.Task | None = None
+
+
+async def periodic_price_fetcher():
+    """Background task that fetches BTC/EUR price every minute."""
+    logger.info(
+        "Starting periodic price fetcher (every %d seconds)", PRICE_REFRESH_SECONDS
+    )
+    while True:
+        try:
+            price_value, timestamp = await fetch_btc_eur_price()
+            async with async_session() as db:
+                new_price = PriceHistory(
+                    source=SOURCE_BITFINEX,
+                    pair=PAIR_BTC_EUR,
+                    price=price_value,
+                    timestamp=timestamp,
+                )
+                db.add(new_price)
+                await db.commit()
+            logger.info("Fetched BTC/EUR price: €%.2f", price_value)
+        except Exception as e:
+            logger.error("Failed to fetch price: %s", e)
+
+        await asyncio.sleep(PRICE_REFRESH_SECONDS)
+
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Create database tables on startup and validate constants."""
+    global _price_fetch_task
+
     # Validate shared constants match backend definitions
     validate_shared_constants()
 
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
+
+    # Start background price fetcher
+    _price_fetch_task = asyncio.create_task(periodic_price_fetcher())
+
     yield
 
+    # Cancel background task on shutdown
+    if _price_fetch_task:
+        _price_fetch_task.cancel()
+        try:
+            await _price_fetch_task
+        except asyncio.CancelledError:
+            logger.info("Price fetcher task cancelled")
+
 
 app = FastAPI(lifespan=lifespan)
 

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),

backend/tests/test_exchange.py

@@ -98,8 +98,7 @@ class TestExchangePriceEndpoint:
         assert "config" in data
         assert data["price"]["market_price"] == 20000.0
         assert data["price"]["premium_percentage"] == 5
-        # Agreed price should be market * 1.05 (5% premium)
-        assert data["price"]["agreed_price"] == pytest.approx(21000.0, rel=0.001)
+        # Note: agreed_price is calculated on frontend based on direction (buy/sell)
 
     @pytest.mark.asyncio
     async def test_admin_cannot_get_price(self, client_factory, admin_user):