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)