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/dashboard_test.go

@@ -46,6 +46,10 @@ type fakeControlAPI struct {
 	// autoCycleDown makes all four endpoints answer 500 (unavailable API).
 	autoCycle     autoCycleDTO
 	autoCycleDown bool
+
+	// authHeaders records the Authorization header of every request served.
+	authMu      sync.Mutex
+	authHeaders []string
 }
 
 func newFakeControlAPI(t *testing.T) (*fakeControlAPI, string) {
@@ -532,7 +536,12 @@ func (f *fakeControlAPI) handler() http.Handler {
 		writeJSON(w, http.StatusOK, map[string]bool{"ok": true})
 	})
 
-	return mux
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		f.authMu.Lock()
+		f.authHeaders = append(f.authHeaders, r.Header.Get("Authorization"))
+		f.authMu.Unlock()
+		mux.ServeHTTP(w, r)
+	})
 }
 
 func (f *fakeControlAPI) findIP(addr string) int {