Add an httpreq endpoint for lego and Traefik

jarld can now answer lego's httpreq DNS provider
(POST /api/v1/acme/httpreq/present and /cleanup), so lego and Traefik
publish DNS-01 challenges with only environment variables, without the
jarl CLI. lego's username and API token arrive as HTTP Basic auth,
accepted on these routes only.

The endpoint is off by default (acme.httpreq.enabled) and HTTPS-only by
default: jarld's own TLS, or a trusted proxy that says so with
X-Forwarded-Proto or Forwarded (the last value counts). Plain HTTP can be
allowed from loopback only or everywhere. GET /api/v1/acme/httpreq shows
how jarld sees a request, to check a proxy setup.

jarl acme and the endpoint share their code in internal/acme. Documented
in docs/acme.md, configuration.md, api.md and permissions.md.

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

CHANGELOG.md

@@ -24,6 +24,20 @@ The first version, to be released as 0.1.0.
   the web UI. Networks smaller than a /24 explain that they belong in the
   enclosing /24's zone.
 - docs/operations.md: setting up a secondary server with its own jarld.
+- An endpoint for lego's `httpreq` DNS provider
+  (`/api/v1/acme/httpreq/present` and `/cleanup`), so lego and Traefik can
+  publish DNS-01 challenges with only environment variables, without the
+  jarl CLI. lego's username and API token arrive as HTTP Basic auth, which
+  is accepted on these routes only. Off by default
+  (`acme.httpreq.enabled`), and HTTPS-only by default: jarld's own TLS, or a
+  trusted proxy that says so with `X-Forwarded-Proto` or `Forwarded`.
+  Plain HTTP can be allowed from loopback only (`allow_loopback_http`) or
+  everywhere (`require_https: false`). `GET /api/v1/acme/httpreq` shows how
+  jarld sees a request, to check a proxy setup. `jarl acme` and the endpoint
+  share their code (`internal/acme`).
+- Trusted proxies may also send the standard `Forwarded: proto=https`
+  header. When a proxy header lists several values, the last one (added by
+  the nearest proxy) counts.
 - lego support: `contrib/lego/jarl-lego.sh` is a program for lego's `exec`
   DNS provider that publishes DNS-01 challenges through `jarl acme`,
   including challenge names reached through a CNAME (docs/acme.md).