Add validation to prevent booking two trades on the same day

backend/tests/test_exchange.py

@@ -260,6 +260,43 @@ class TestCreateExchange:
         assert response.status_code == 409
         assert "already been booked" in response.json()["detail"]
 
+    @pytest.mark.asyncio
+    async def test_cannot_book_two_trades_same_day(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Cannot book two trades on the same day."""
+        target_date = await setup_availability_and_price(client_factory, admin_user)
+
+        with mock_price_fetcher(20000.0):
+            # First trade
+            async with client_factory.create(cookies=regular_user["cookies"]) as client:
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "onchain",
+                        "eur_amount": 10000,
+                    },
+                )
+            assert response.status_code == 200
+
+            # Try to book another trade on the same day
+            async with client_factory.create(cookies=regular_user["cookies"]) as client:
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T10:00:00Z",
+                        "direction": "sell",
+                        "bitcoin_transfer_method": "lightning",
+                        "eur_amount": 20000,
+                    },
+                )
+
+        assert response.status_code == 400
+        assert "already have a trade booked" in response.json()["detail"]
+        assert "Trade ID:" in response.json()["detail"]
+
     @pytest.mark.asyncio
     async def test_invalid_direction_rejected(
         self, client_factory, regular_user, admin_user