Add PayerEin (nullable string, max 20) to ChurchProfile entity, AppDbContext
config, ChurchProfileDto response, UpdateChurchProfileRequest, and
ChurchProfileService round-trip — mirroring the Phone/BankRoutingNumber
nullable-string pattern. Migration AddPayerEinToChurchProfile adds only the
one nullable column to ChurchProfiles.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>