Add Lightning amount threshold validation

backend/routes/exchange.py

@@ -33,6 +33,7 @@ from shared_constants import (
     EUR_TRADE_INCREMENT,
     EUR_TRADE_MAX,
     EUR_TRADE_MIN,
+    LIGHTNING_MAX_EUR,
     PREMIUM_PERCENTAGE,
     PRICE_STALENESS_SECONDS,
     SLOT_DURATION_MINUTES,
@@ -415,6 +416,20 @@ async def create_exchange(
             detail=f"EUR amount must be a multiple of €{EUR_TRADE_INCREMENT}",
         )
 
+    # Validate Lightning threshold
+    if (
+        bitcoin_transfer_method == BitcoinTransferMethod.LIGHTNING
+        and request.eur_amount > LIGHTNING_MAX_EUR * 100
+    ):
+        raise HTTPException(
+            status_code=400,
+            detail=(
+                f"Lightning payments are only allowed for amounts up to "
+                f"€{LIGHTNING_MAX_EUR}. For amounts above €{LIGHTNING_MAX_EUR}, "
+                "please use onchain transactions."
+            ),
+        )
+
     # Validate slot timing - compute valid boundaries from slot duration
     valid_minutes = tuple(range(0, 60, SLOT_DURATION_MINUTES))
     if request.slot_start.minute not in valid_minutes:

backend/shared_constants.py

@@ -16,3 +16,4 @@ EUR_TRADE_INCREMENT: int = _constants["exchange"]["eurTradeIncrement"]
 PREMIUM_PERCENTAGE: int = _constants["exchange"]["premiumPercentage"]
 PRICE_REFRESH_SECONDS: int = _constants["exchange"]["priceRefreshSeconds"]
 PRICE_STALENESS_SECONDS: int = _constants["exchange"]["priceStalenessSeconds"]
+LIGHTNING_MAX_EUR: int = _constants["exchange"]["lightningMaxEur"]

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