TLS certificates: Jarl obtains and renews them over ACME

Jarl is now an ACME client (lego's core, no DNS providers). It answers
DNS-01 challenges in a zone it serves, or through Porkbun's DNS API while
Porkbun serves the domain; "auto" decides from the domain's public name
servers at every renewal, and a CNAME on _acme-challenge is followed.
Certificates are used for jarld's HTTPS (served without a restart; each
port speaks HTTPS and plain HTTP, plain HTTP from loopback and trusted
proxies still served) and/or written for named's DNS over TLS/HTTPS, with
rndc reconfig on renewal. Renewal follows the CA's ARI window or two
thirds of the lifetime, with back-off. Certificates you renew yourself
can be used too. Certificates page and dashboard card, jarl cert,
/api/v1/certificates, Prometheus gauges, docs/certificates.md, and an
e2e section against Pebble.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/acme.md

@@ -1,5 +1,11 @@
 # Let's Encrypt (ACME DNS-01) with Jarl
 
+This page is about **other** ACME clients (certbot, acme.sh, lego,
+Traefik) publishing their DNS-01 challenges through Jarl. For
+certificates for the DNS server itself (Jarl's web UI, named's DNS over
+TLS and HTTPS), Jarl can be the ACME client: see
+[certificates.md](certificates.md).
+
 The DNS-01 challenge proves that you control a domain by publishing a TXT
 record at `_acme-challenge.<domain>`. Jarl does this through its API, so no
 dynamic updates (`allow-update`, nsupdate) are needed. That matters because