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

backend/validate_constants.py

@@ -75,10 +75,7 @@ def validate_shared_constants() -> None:
         "slotDurationMinutes",
         "maxAdvanceDays",
         "minAdvanceDays",
-        "eurTradeMin",
-        "eurTradeMax",
         "eurTradeIncrement",
-        "premiumPercentage",
         "priceRefreshSeconds",
         "priceStalenessSeconds",
         "lightningMaxEur",