Adds a `credentials` entity to the back office (never mirrored to
Sheets, gated by the CRM token even to read) so client logins like
the auto-generated Easy!Appointments provider password can be viewed
and copied from the dashboard instead of getting lost — the actual
cause of the happynails password going missing. Onboarding now saves
that generated password instead of discarding it. Also adds
Documentation.md, brings README/TODO in line with the current
Postgres-first architecture, and tidies the backlog.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
mivanchenko