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>

rxprod-compose/docker-compose.yml

@@ -28,6 +28,10 @@ services:
       OS_USERNAME: "${OS_USERNAME:-}"
       OS_USER_DOMAIN_NAME: "${OS_USER_DOMAIN_NAME:-}"
       OS_PASSWORD: "${OS_PASSWORD:-}"
+      # Static bearer tokens (set in .env; empty = that API level is open).
+      # Generate with: openssl rand -hex 32
+      CONTROL_API_ADMIN_TOKEN: "${CONTROL_API_ADMIN_TOKEN:-}"
+      CONTROL_API_AGENT_TOKEN: "${CONTROL_API_AGENT_TOKEN:-}"
     healthcheck:
       test: ["CMD", "wget", "--quiet", "--tries=1", "--spider", "http://127.0.0.1:8081/healthz"]
       interval: 5s
@@ -44,6 +48,12 @@ services:
       - "8091:8090"
     environment:
       ADMIN_DASHBOARD_CONTROL_API_URL: http://control-api:8081
+      # Admin token of control-api + the dashboard login (empty username/
+      # password = login disabled).
+      ADMIN_DASHBOARD_CONTROL_API_TOKEN: "${ADMIN_DASHBOARD_CONTROL_API_TOKEN:-}"
+      ADMIN_DASHBOARD_USERNAME: "${ADMIN_DASHBOARD_USERNAME:-}"
+      ADMIN_DASHBOARD_PASSWORD: "${ADMIN_DASHBOARD_PASSWORD:-}"
+      ADMIN_DASHBOARD_SESSION_SECRET: "${ADMIN_DASHBOARD_SESSION_SECRET:-}"
     depends_on:
       control-api:
         condition: service_healthy
@@ -57,6 +67,7 @@ services:
     environment:
       PROBER_SITE_ID: "${PROBER_SITE_ID}"
       PROBER_CONTROL_API_URL: http://control-api:8081
+      CONTROL_API_AGENT_TOKEN: "${CONTROL_API_AGENT_TOKEN:-}"
     depends_on:
       control-api:
         condition: service_healthy