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>

cmd/jarl/certs.go

@@ -99,9 +99,10 @@ used too (--cert-file and --key-file): Jarl rereads it when it changes.`,
   jarl cert create own --cert-file /etc/ssl/ns1/fullchain.pem --key-file /etc/ssl/ns1/privkey.pem --use web
   jarl cert list
   jarl cert issue ns1                                      # renew now
-  jarl cert update ns1 --challenge porkbun                 # pin the route (auto, jarl or porkbun)`,
+  jarl cert update ns1 --challenge porkbun                 # pin the route (auto, jarl or porkbun)
+  jarl cert export ns1 --dir /etc/ssl/ns1                  # fullchain.pem and privkey.pem, for another program`,
 	}
-	cmd.AddCommand(certListCmd(), certShowCmd(), certCreateCmd(), certUpdateCmd(), certIssueCmd(), certCheckCmd(), certDeleteCmd(), certAccountCmd())
+	cmd.AddCommand(certListCmd(), certShowCmd(), certCreateCmd(), certUpdateCmd(), certIssueCmd(), certCheckCmd(), certExportCmd(), certDeleteCmd(), certAccountCmd())
 	return cmd
 }
 
@@ -493,6 +494,91 @@ is published and the CA isn't asked.`,
 	return cmd
 }
 
+func certExportCmd() *cobra.Command {
+	var dir, part string
+	var noKey, force bool
+	cmd := &cobra.Command{
+		Use:   "export NAME",
+		Short: "Save a certificate (and its key) to use in another program",
+		Long: `Saves a certificate's current files, to use it outside Jarl (a mail
+server, a reverse proxy): fullchain.pem and privkey.pem in --dir (the
+key readable only by you), or one part on standard output with --part:
+
+  fullchain  the certificate and its chain
+  cert       the certificate alone
+  key        the private key
+  bundle     the key, then the chain (one file, as HAProxy wants)
+
+Administrators only; every export of the key is audited. Jarl renews the
+certificate, but not your copy: export it again after a renewal (the
+'renews' date in 'jarl cert show'), or point the program at the files Jarl
+writes for named (use dns) instead.`,
+		Example: `  jarl cert export ns1 --dir /etc/ssl/ns1
+  jarl cert export ns1 --part bundle > /etc/haproxy/ns1.pem
+  jarl cert export ns1 --no-key --dir .`,
+		Args: cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newClient()
+			if err != nil {
+				return err
+			}
+			get := func(p string) (string, error) {
+				var out string
+				err := c.Get(ctx(), certPath(args[0])+"/pem", url.Values{"part": {p}}, &out)
+				return out, err
+			}
+			if part != "" {
+				out, err := get(part)
+				if err != nil {
+					return err
+				}
+				_, err = os.Stdout.WriteString(out)
+				return err
+			}
+			files := []struct{ part, name string }{{"fullchain", "fullchain.pem"}, {"key", "privkey.pem"}}
+			if noKey {
+				files = files[:1]
+			}
+			if err := os.MkdirAll(dir, 0o755); err != nil {
+				return err
+			}
+			for _, f := range files {
+				path := dir + string(os.PathSeparator) + f.name
+				if _, err := os.Stat(path); err == nil && !force {
+					return fmt.Errorf("%s exists (--force replaces it)", path)
+				}
+			}
+			for _, f := range files {
+				out, err := get(f.part)
+				if err != nil {
+					return err
+				}
+				path := dir + string(os.PathSeparator) + f.name
+				mode := os.FileMode(0o644)
+				if f.part == "key" {
+					mode = 0o600
+				}
+				if err := os.WriteFile(path, []byte(out), mode); err != nil {
+					return err
+				}
+				if err := os.Chmod(path, mode); err != nil {
+					return err
+				}
+				if !flagQuiet {
+					fmt.Println("wrote", path)
+				}
+			}
+			return nil
+		},
+	}
+	cmd.Flags().StringVar(&dir, "dir", ".", "directory for fullchain.pem and privkey.pem")
+	cmd.Flags().StringVar(&part, "part", "", "write one part to standard output: fullchain, cert, key or bundle")
+	cmd.Flags().BoolVar(&noKey, "no-key", false, "only fullchain.pem")
+	cmd.Flags().BoolVar(&force, "force", false, "replace existing files")
+	_ = cmd.RegisterFlagCompletionFunc("part", cobra.FixedCompletions([]string{"fullchain", "cert", "key", "bundle"}, cobra.ShellCompDirectiveNoFileComp))
+	return cmd
+}
+
 func certDeleteCmd() *cobra.Command {
 	var yes bool
 	cmd := &cobra.Command{