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>

internal/engine/files.go

@@ -0,0 +1,22 @@
+package engine
+
+import (
+	"context"
+	"io/fs"
+)
+
+// WriteFile writes a file outside the rendered configuration (a
+// certificate for named, docs/certificates.md) the way deployed files are
+// written: atomically, with bind.file_group, creating its directory.
+func (e *Engine) WriteFile(path string, content []byte, mode fs.FileMode) error {
+	return e.deploy.writeAtomic(path, content, mode)
+}
+
+// Reconfig makes named reread its configuration, and with it the files
+// its tls blocks name (nothing with bind.no_reload).
+func (e *Engine) Reconfig(ctx context.Context) error {
+	if e.cfg.NoReload {
+		return nil
+	}
+	return e.tools.Reconfig(ctx)
+}