Foundation for DB-backed user management (#1605):
- UserRecord, ApiTokenRecord, InvitationRecord types in db/mod.rs
- UserStore sub-trait (17 methods) added to Database supertrait
- PostgreSQL migration V14__users.sql (users, api_tokens, invitations)
- libSQL schema + incremental migration V14
- Full implementations for both PgBackend (via Store delegation) and
LibSqlBackend (direct SQL in libsql/users.rs)
- authenticate_token JOINs api_tokens+users with active/non-revoked
checks; has_any_users for bootstrap detection
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>
