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>

internal/dashboard/templates/layout.html

@@ -78,6 +78,13 @@
 </a>
 </nav>
 
+{{if .AuthEnabled}}
+<form class="sidebar-user" method="post" action="/logout">
+<span class="sidebar-user-name" title="{{.User}}">{{.User}}</span>
+<button type="submit" class="btn btn-ghost btn-sm">Выйти</button>
+</form>
+{{end}}
+
 <div class="sidebar-foot">
 <span class="sidebar-foot-status"><span class="pulse-dot"></span>control-api</span>
 <button type="button" class="theme-toggle" id="themeToggle" aria-label="Переключить тему" title="Переключить тему">

internal/dashboard/templates/login.html

@@ -0,0 +1,29 @@
+{{define "login_page"}}
+<!doctype html>
+<html lang="ru">
+<head>{{template "html_head" .}}</head>
+<body>
+<div class="bg-grid"></div>
+<main class="login-shell">
+<div class="login-card">
+<form class="panel" method="post" action="/login" autocomplete="on">
+<div class="panel-head"><h2>Cloud IP Validator — вход</h2></div>
+<div class="panel-body">
+{{if .Error}}<div class="alert alert-warning" role="alert">{{.Error}}</div>{{end}}
+<input type="hidden" name="next" value="{{.Next}}">
+<div class="field">
+<label for="login-username">Логин</label>
+<input type="text" id="login-username" name="username" autocomplete="username" autofocus required>
+</div>
+<div class="field">
+<label for="login-password">Пароль</label>
+<input type="password" id="login-password" name="password" autocomplete="current-password" required>
+</div>
+<button type="submit" class="btn btn-primary btn-block">Войти</button>
+</div>
+</form>
+</div>
+</main>
+</body>
+</html>
+{{end}}