test: Add test for cancelled slot becoming available again

Verifies that when a user cancels their trade, the slot becomes
available for booking by other users.

backend/tests/test_exchange.py

@@ -482,6 +482,49 @@ class TestCancelTrade:
         assert data["status"] == "cancelled_by_user"
         assert data["cancelled_at"] is not None
 
+    @pytest.mark.asyncio
+    async def test_cancelled_slot_becomes_available_again(
+        self, client_factory, regular_user, admin_user
+    ):
+        """When a trade is cancelled, the slot becomes available for booking again."""
+        target_date = await setup_availability_and_price(client_factory, admin_user)
+
+        with mock_price_fetcher(20000.0):
+            async with client_factory.create(cookies=regular_user["cookies"]) as client:
+                # Book a slot
+                book_response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "eur_amount": 10000,
+                    },
+                )
+                assert book_response.status_code == 200
+                trade_id = book_response.json()["id"]
+
+                # Verify the slot is NOT available
+                slots_response = await client.get(
+                    f"/api/exchange/slots?date={target_date}"
+                )
+                assert slots_response.status_code == 200
+                slots = slots_response.json()["slots"]
+                slot_starts = [s["start_time"] for s in slots]
+                assert f"{target_date}T09:00:00Z" not in slot_starts
+
+                # Cancel the trade
+                cancel_response = await client.post(f"/api/trades/{trade_id}/cancel")
+                assert cancel_response.status_code == 200
+
+                # Verify the slot IS available again
+                slots_response = await client.get(
+                    f"/api/exchange/slots?date={target_date}"
+                )
+                assert slots_response.status_code == 200
+                slots = slots_response.json()["slots"]
+                slot_starts = [s["start_time"] for s in slots]
+                assert f"{target_date}T09:00:00Z" in slot_starts
+
     @pytest.mark.asyncio
     async def test_cannot_cancel_others_trade(
         self, client_factory, regular_user, alt_regular_user, admin_user