Certificate export, missing PTR list, simpler "Used for"

- Certificates can be exported for use outside Jarl: Export… on the
  Certificates page (download or copy), jarl cert export, and
  GET /certificates/{name}/pem?part=fullchain|cert|key|bundle.
  Administrators only; key exports are audited.
- A reverse zone's missing-PTR banner lists the addresses and names
  (ptr_missing_list in the zone API).
- "Used for" shows DoT/DoH for named, and "not used by Jarl" instead of
  "nothing yet".

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

internal/certs/manager.go

@@ -295,6 +295,31 @@ func (m *Manager) Get(ctx context.Context, name string) (*model.Certificate, err
 	return c, err
 }
 
+// PEM returns a certificate's current chain and private key, for
+// exporting it: from Jarl's store, or from the files of one you renew
+// yourself.
+func (m *Manager) PEM(ctx context.Context, name string) (*model.Certificate, []byte, []byte, error) {
+	c, err := m.Get(ctx, name)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+	if c.Source == model.CertSourceFile {
+		cert, err := os.ReadFile(c.CertFile)
+		if err != nil {
+			return nil, nil, nil, fileErr(err)
+		}
+		key, err := os.ReadFile(c.KeyFile)
+		if err != nil {
+			return nil, nil, nil, fileErr(err)
+		}
+		return c, cert, key, nil
+	}
+	if c.CertPEM == "" {
+		return nil, nil, nil, &InputError{Msg: "certificate " + name + " hasn't been obtained yet, so there is nothing to export"}
+	}
+	return c, []byte(c.CertPEM), []byte(c.KeyPEM), nil
+}
+
 // Create adds a certificate. A file-source certificate is read at once;
 // one Jarl obtains is left for Issue (or the background loop).
 func (m *Manager) Create(ctx context.Context, in Input, p *auth.Principal) (*model.Certificate, error) {