fix: rewrite seed SQL to match actual IMS database schema

Previous seed assumed columns that don't exist (IsEnabled,
IsAnEReturnDefaultFund). DemoData.sql already seeds 19 funds, MOPs,
VAT codes, templates, suspense items. Custom seed now only adds:
account holders, transactions, extra users, fund groups, GOV.UK Pay
API keys. All column names verified against upstream entity classes.

Author: chrisns