Eight tasks to put a session gate in front of every page and API route: the auth primitives
(PBKDF2 verifier, HMAC-signed stateless session, session-bound CSRF token, login throttle,
origin check) in a new `auth.py`, the verifier in the store plus fail-closed startup, the
default-deny middleware with an explicit public list, `POST /api/auth/password` with session
rotation, a login page, a `/settings` page, and the dev-config/docs/browser verification pass.
Stdlib hashlib/hmac only — no new dependency, so no PLAN.md anchor moves. `/api/health`
stays public on purpose (the Containerfile HEALTHCHECK and dev-server.sh depend on it), and
one test enumerates every route and demands a session.
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