Add authentication: admin/agent bearer tokens for the API, login for the dashboard

control-api: every route now carries a mandatory access level (admin / agent /
open) in a route table. All /api/v1/admin/* require the admin token; the
write calls of validator-agent and prober (self-check, events, results,
complete) require a separate static agent token; register, heartbeat and
fetching the assignment stay open. Tokens come from env vars, are compared in
constant time and never logged. An empty token leaves that level open with a
startup warning (backward compatible).

validator-agent / prober: apiclient sends the agent token only to control-api.

admin-dashboard: login/password (from env) with a stateless HMAC session
cookie, Origin-based CSRF check, per-IP brute-force throttle, HX-Redirect for
htmx polls, logout in the sidebar; the dashboard calls control-api with the
admin token. Login page layout fixed after review.

Also: env plumbing in docker-compose/rxprod-compose/systemd/config examples,
e2e script with token assertions, tests, docs (API, SETUP, USAGE, DASHBOARD,
README), plan and review under docs/changes/, bin/ rebuilt with new
SHA256SUMS.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/USAGE.md

@@ -7,6 +7,12 @@
 [API.md](API.md) — здесь мы используем их только как инструмент, не
 углубляясь в протокол.
 
+> **Токен в примерах.** Если на стенде включена аутентификация
+> ([SETUP.md](SETUP.md#5-аутентификация-токены-и-пароль-дашборда)), к вызовам
+> `/api/v1/admin/*` в примерах `curl` ниже нужно добавлять заголовок
+> `-H "Authorization: Bearer $ADMIN_TOKEN"` (значение `CONTROL_API_ADMIN_TOKEN`);
+> для краткости он опущен. Дашборд запрашивает логин и пароль.
+
 ## Содержание
 
 - [Как устроена работа с системой](#как-устроена-работа-с-системой)