Certificates for other hosts: jarl cert get

jarld obtains and renews a certificate for another host; the host asks
for it with one command, waits, downloads it when it is newer than its
copy and runs a hook with the paths in JARL_CERT_* variables.

- Certificates have an owner user; another user can't take the name.
  An existing one is obtained again only when a name is added or
  removed. A new certificates grant (names=) says which names a user
  may have; it is never part of "*". Users may own
  certificates.max_per_user certificates (10) or their own cert_limit.
- POST /certificates/{name}/request; owners may list, show, export and
  delete theirs; keys only over HTTPS, the socket or loopback; every
  download is recorded and shown as "Pulled by".
- CLI: jarl cert get (saved settings, all saved when no name), cert
  saved, cert forget, cert schedule (daily systemd timer, Linux, root).
- Migration 14. Tests: server, auth, and e2e against Pebble.

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

internal/model/changeset.go

@@ -76,6 +76,9 @@ type User struct {
 	TOTPSecret    string     `json:"-"`
 	TOTPPending   string     `json:"-"`
 	TOTPLastStep  int64      `json:"-"`
+	// CertLimit is how many certificates the user may own (jarl cert get);
+	// nil: certificates.max_per_user in jarl.yaml.
+	CertLimit *int `json:"cert_limit,omitempty"`
 }
 
 // HasTOTP reports whether the user logs in with a TOTP code.