Add credentials store to the CRM, docs cleanup, deploy pipeline TODO

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>

backoffice/app/import_from_sheets.py

@@ -44,6 +44,8 @@ def main():
         with conn.cursor() as cur:
             cur.execute("TRUNCATE activity_log RESTART IDENTITY")
             for entity, spec in db.TABLES.items():
+                if spec.get("mirror") is False:
+                    continue  # e.g. credentials — never lived in the Sheet, nothing to import
                 records = sh.read_records(spec["tab"])
                 n = upsert(cur, entity, records)
                 print(f"  {entity:<13} <- {spec['tab']:<13} {n} row(s)")