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>

configs/validator-agent.example.yaml

@@ -3,6 +3,12 @@
 
 validator_id: "validator_01"
 control_api_url: "http://control-api.internal:8080"
+# Имя переменной окружения с токеном агентов control-api (тот же, что у
+# prober; CONTROL_API_AGENT_TOKEN на стороне control-api). Нужен для записи
+# результатов/событий; register/heartbeat/получение задания работают без него.
+# Токен отправляется только в control-api — не на ip_echo_urls и не на цели
+# проверок.
+control_api_token_env: "CONTROL_API_AGENT_TOKEN"
 poll_interval_seconds: 5
 
 self_check: