Adds DB triggers + backfill so accounting.customers is driven by the
public units/owners roster: one customer per unit, all owners combined,
Units/Owners as source of truth for contact fields. Balances and ledger
links (invoices, payments_received, transactions, work_orders, estimates)
are always preserved. Scoped to associations with an accounting company.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
