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/docker-compose.yml

@@ -34,6 +34,11 @@ services:
     networks: [backend]
     volumes:
       - control-api-db:/var/lib/cloud-ip-validator
+    environment:
+      # Static bearer tokens (empty = that API level stays open, with a
+      # warning in the log). 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://127.0.0.1:8080/healthz"]
       interval: 5s
@@ -55,6 +60,12 @@ services:
       ADMIN_DASHBOARD_CONTROL_API_TIMEOUT_SECONDS: "${ADMIN_DASHBOARD_CONTROL_API_TIMEOUT_SECONDS:-10}"
       ADMIN_DASHBOARD_LAST_COMPLETED_COUNT: "${ADMIN_DASHBOARD_LAST_COMPLETED_COUNT:-20}"
       ADMIN_DASHBOARD_POLL_INTERVAL_SECONDS: "${ADMIN_DASHBOARD_POLL_INTERVAL_SECONDS:-5}"
+      # Admin token of control-api, and the dashboard's own login. Empty
+      # username/password = login disabled (warning in the log).
+      ADMIN_DASHBOARD_CONTROL_API_TOKEN: "${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:-}"
 
   prober:
     build:
@@ -72,6 +83,8 @@ services:
       PROBER_TCP_TIMEOUT_SECONDS: "${PROBER_TCP_TIMEOUT_SECONDS:-5}"
       PROBER_ICMP_TIMEOUT_SECONDS: "${PROBER_ICMP_TIMEOUT_SECONDS:-5}"
       PROBER_ICMP_COUNT: "${PROBER_ICMP_COUNT:-3}"
+      # Agent token of control-api (same value as on control-api).
+      CONTROL_API_AGENT_TOKEN: "${CONTROL_API_AGENT_TOKEN:-}"
 
   validator-agent:
     build:
@@ -92,3 +105,5 @@ services:
       VALIDATOR_AGENT_ICMP_COUNT: "${VALIDATOR_AGENT_ICMP_COUNT:-3}"
       VALIDATOR_AGENT_SSH_ENABLED: "${VALIDATOR_AGENT_SSH_ENABLED:-false}"
       VALIDATOR_AGENT_SSH_TIMEOUT_SECONDS: "${VALIDATOR_AGENT_SSH_TIMEOUT_SECONDS:-5}"
+      # Agent token of control-api (same value as on control-api).
+      CONTROL_API_AGENT_TOKEN: "${CONTROL_API_AGENT_TOKEN:-}"