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>

cmd/control-api/main.go

@@ -60,7 +60,16 @@ func run(configPath string, log *slog.Logger) error {
 
 	orch := orchestrator.New(database, osClient, cfg, log)
 
-	srv := httpapi.New(database, orch, log)
+	adminToken := os.Getenv(cfg.Auth.AdminTokenEnv)
+	agentToken := os.Getenv(cfg.Auth.AgentTokenEnv)
+	if adminToken == "" {
+		log.Warn("admin API is open: admin token is not set", "env", cfg.Auth.AdminTokenEnv)
+	}
+	if agentToken == "" {
+		log.Warn("agent write API is open: agent token is not set", "env", cfg.Auth.AgentTokenEnv)
+	}
+
+	srv := httpapi.New(database, orch, log).WithAuth(adminToken, agentToken)
 	httpServer := &http.Server{Addr: cfg.Server.ListenAddr, Handler: srv.Handler()}
 
 	go runOrchestratorLoop(ctx, orch, cfg, log)