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: