fix: Prevent user from cancelling trades after slot time has passed

Users can no longer cancel trades once the slot time has passed.
Added test to verify this behavior.

backend/tests/test_exchange.py

@@ -543,6 +543,39 @@ class TestCancelTrade:
         assert response.status_code == 400
         assert "cancelled_by_user" in response.json()["detail"]
 
+    @pytest.mark.asyncio
+    async def test_cannot_cancel_past_trade(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Cannot cancel a trade after the slot time has passed."""
+        # Create a past trade directly in DB
+        async with client_factory.get_db_session() as db:
+            await create_price_in_db(db, price=20000.0)
+            past_time = datetime.now(UTC) - timedelta(hours=1)
+            exchange = Exchange(
+                user_id=regular_user["user"]["id"],
+                slot_start=past_time,
+                slot_end=past_time + timedelta(minutes=15),
+                direction=TradeDirection.BUY,
+                eur_amount=10000,
+                sats_amount=500000,
+                market_price_eur=20000.0,
+                agreed_price_eur=21000.0,
+                premium_percentage=5,
+                status=ExchangeStatus.BOOKED,
+            )
+            db.add(exchange)
+            await db.commit()
+            await db.refresh(exchange)
+            trade_id = exchange.id
+
+        # User tries to cancel
+        async with client_factory.create(cookies=regular_user["cookies"]) as client:
+            response = await client.post(f"/api/trades/{trade_id}/cancel")
+
+        assert response.status_code == 400
+        assert "already passed" in response.json()["detail"]
+
 
 # =============================================================================
 # Admin Trades Tests