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>

CHANGELOG.md

@@ -40,6 +40,28 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Added
 
+- TLS certificates ([docs/certificates.md](docs/certificates.md)): Jarl
+  obtains certificates from Let's Encrypt or any ACME CA (account set up
+  in jarl.yaml under `acme.account`) and renews them itself, from the CA's
+  renewal information (ARI) or two thirds into their lifetime, with
+  back-off after failures. It answers the 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,
+  so moving the domain to your own name servers needs no change. A CNAME
+  on `_acme-challenge` into a zone Jarl serves is followed, for domains
+  hosted elsewhere. A certificate is used for jarld's HTTPS ("web") and/or
+  written for named (`<bind.config_dir>/tls/NAME/`, "dns"; named rereads
+  it on renewal), and certificates you renew yourself can be used too
+  (reread when their files change). Certificates page and dashboard card
+  in the web UI, `jarl cert` (check, create, issue, update, delete,
+  account), `/api/v1/certificates`, Prometheus
+  `jarl_certificate_expiry_timestamp_seconds`, `jarl_certificate_failing`.
+- jarld serves HTTPS and plain HTTP on the same ports once it has a
+  certificate (`tls.cert_file`, or one with the web use, which takes
+  effect without a restart): plain HTTP from the server itself and from
+  trusted proxies is served as before, browsers elsewhere are redirected
+  to HTTPS and API requests get `https_required`. Before, with
+  `tls.cert_file`, plain HTTP got an error even from localhost.
 - Internet exposure ([docs/exposure.md](docs/exposure.md)): Jarl works out
   which views are reachable from the internet (match-clients ending in a
   catch-all; for a server without views, open allow-query and a