Aligns the auth-account table name with the AppUser entity. The historical
mismatch (table 'users' alongside table 'persons') misled schema-first readers
into assuming the two were related; renaming to 'app_users' makes the
deliberate split between auth accounts and historical persons explicit at the
schema layer.
Scope: the table itself, the users_groups join table, and the three FK columns
whose name was literally 'user_id'. Semantic FK columns (audit_log.actor_id,
notifications.recipient_id, document_versions.editor_id, etc.) keep their
names — the role they describe is the documentation, not the type.
Closes#418. Unblocks #407 (REFACTOR-1).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
