Step 8: Cleanup old constants

- Remove EUR_TRADE_MIN, EUR_TRADE_MAX, PREMIUM_PERCENTAGE from shared_constants.py
- Remove eurTradeMin, eurTradeMax, premiumPercentage from shared/constants.json
- Update validate_constants.py to not require removed fields
- Update seed.py and seed_e2e.py to use defaults if fields don't exist
- Update tests to handle missing constants gracefully

shared/constants.json

@@ -27,10 +27,7 @@
     "slotDurationMinutes": 15,
     "maxAdvanceDays": 30,
     "minAdvanceDays": 1,
-    "eurTradeMin": 100,
-    "eurTradeMax": 3000,
     "eurTradeIncrement": 20,
-    "premiumPercentage": 5,
     "priceRefreshSeconds": 60,
     "priceStalenessSeconds": 300,
     "lightningMaxEur": 1000