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>

deploy/docker/prober/docker-entrypoint.sh

@@ -9,6 +9,10 @@ export PROBER_TCP_TIMEOUT_SECONDS="${PROBER_TCP_TIMEOUT_SECONDS:-5}"
 export PROBER_ICMP_TIMEOUT_SECONDS="${PROBER_ICMP_TIMEOUT_SECONDS:-5}"
 export PROBER_ICMP_COUNT="${PROBER_ICMP_COUNT:-3}"
 
+# Tokens/passwords (CONTROL_API_AGENT_TOKEN, ADMIN_DASHBOARD_*) are deliberately
+# NOT templated into the YAML: the binary reads them straight from this
+# container's environment (names default in the config loader), so secrets
+# never land in a file inside the container.
 envsubst '${PROBER_SITE_ID} ${PROBER_CONTROL_API_URL} ${PROBER_POLL_INTERVAL_SECONDS} ${PROBER_TCP_TIMEOUT_SECONDS} ${PROBER_ICMP_TIMEOUT_SECONDS} ${PROBER_ICMP_COUNT}' \
     < /etc/prober/prober.yaml.tmpl > /etc/prober/prober.yaml