Adds identity.py (schema, capability map, scrypt passwords, invites,
sessions, login throttle, boot seed) and auth.py (login, invite
acceptance, account page). app.py gains two imports and one wiring
block at EOF; no existing behaviour changes.
Units are a table seeded from the site constants. Roles split: leader
and member per unit in memberships, owner and admin site-wide in
people.global_role, so a unit added later cannot under-grant an admin.
tests/smoke_identity.py covers the rules that are invisible when wrong:
single-use invites, reissue revoking the prior link, expiry, idle and
absolute session bounds, throttling, and the capability split. 49 checks.
