fix: derive valid slot minutes from SLOT_DURATION_MINUTES constant

Replace hardcoded (0, 15, 30, 45) tuple with computed range based on
the SLOT_DURATION_MINUTES constant. This ensures the validation stays
in sync if the slot duration is ever changed.

Add test to verify slot minute boundary validation.

backend/routes/exchange.py

@@ -401,8 +401,8 @@ async def create_exchange(
             detail=f"EUR amount must be a multiple of €{EUR_TRADE_INCREMENT}",
         )
 
-    # Validate slot timing
-    valid_minutes = (0, 15, 30, 45)
+    # 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:
         raise HTTPException(
             status_code=400,