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>

bin/SHA256SUMS

@@ -1,4 +1,4 @@
-fb8aaab7c07c3702d274aa15fcb299690bb738cbde9faad79e73d726c24e9d1b  control-api
-d7377e90cc34549933f0de8556df113f7ace7425ad0fee96f723ed6a4510d66b  validator-agent
-c3c9e42ad7632cd8a4b88fe5eedf341cf0dcba6ecf5281d247733a98cee91fb6  prober
-23d1c10c26366dedd790025689eae63677316c7bb9dafb53c8cebca340dfe77f  admin-dashboard
+a0eac5409a2929d641ef2a217c31f1b6a974a8679da866ebbb50ff1d5b57df8d  control-api
+5d7fb2f476871843ae4179dddb61e77d3f71e8eb73a00e331f0119d8893ba813  validator-agent
+23c687a988350c61f486d400af654bd563865fff59421c437eb679e0e4e10500  prober
+90921ad25f16a922124368770e439d1228c5b0bf7126ba6c726f3fb98bb22f7a  admin-dashboard