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>

go.mod

@@ -4,6 +4,7 @@ go 1.27.1
 
 require (
 	github.com/aymanbagabas/go-udiff v0.4.1
+	github.com/go-acme/lego/v4 v4.35.2
 	github.com/miekg/dns v1.1.73
 	github.com/spf13/cobra v1.10.2
 	go.yaml.in/yaml/v3 v3.0.5
@@ -15,7 +16,9 @@ require (
 )
 
 require (
+	github.com/cenkalti/backoff/v5 v5.0.3 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/mattn/go-isatty v0.0.24 // indirect