Adds two new filters to the activity log:
- Reference search: searches inside JSON old_values/new_values
  for invoice numbers (INV-...) or payment references (PAY-...)
- Entity type filter: quickly narrow to invoices, payments, etc.

Uses PostgreSQL jsonb::text ILIKE for the deep JSON search.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>