Add Lightning amount threshold validation

backend/tests/test_exchange.py

@@ -324,6 +324,77 @@ class TestCreateExchange:
         assert response.status_code == 400
         assert "Invalid bitcoin_transfer_method" in response.json()["detail"]
 
+    @pytest.mark.asyncio
+    async def test_lightning_above_threshold_rejected(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Lightning payment above threshold is rejected."""
+        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:
+                # Try Lightning with amount above threshold (€1000 = 100000 cents)
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "lightning",
+                        "eur_amount": 110000,  # €1100, above €1000 threshold
+                    },
+                )
+
+        assert response.status_code == 400
+        assert "Lightning payments are only allowed" in response.json()["detail"]
+
+    @pytest.mark.asyncio
+    async def test_lightning_below_threshold_accepted(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Lightning payment below threshold is accepted."""
+        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:
+                # Lightning with amount at threshold
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "lightning",
+                        "eur_amount": 100000,  # €1000, exactly at threshold
+                    },
+                )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["bitcoin_transfer_method"] == "lightning"
+
+    @pytest.mark.asyncio
+    async def test_onchain_above_threshold_accepted(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Onchain payment above threshold is accepted."""
+        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:
+                # Onchain with amount above Lightning threshold
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "onchain",
+                        "eur_amount": 110000,  # €1100, above €1000 threshold
+                    },
+                )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["bitcoin_transfer_method"] == "onchain"
+
     @pytest.mark.asyncio
     async def test_amount_below_minimum_rejected(
         self, client_factory, regular_user, admin_user