feat: optional password encryption for backup and domain export (code-review.md § Phase 1.5)

Both secret-bearing downloads can now be sealed with a password. Unticked, the
forms produce exactly the files they did before.

- internal/secretfile: envelope format — magic/type/scrypt params/salt/nonce
  prefix header, then 64 KiB AES-256-GCM chunks each authenticated with the
  header, its counter and an end-of-stream flag, so truncation, reordering and
  tampering fail to open instead of restoring a plausible prefix. Streams both
  ways, so a full backup never sits in memory.
- Panel: "Encrypt with a password" checkbox on the full-backup and
  domain-export forms (shared partial, toggled from panel.js — no inline
  script); domain import detects an encrypted export by magic bytes, not by
  extension, and asks for the password.
- selfpost-backup: writes .spbk when given a password and converts one back
  with -decrypt, which a restore needs. The password comes from
  SELFPOST_BACKUP_PASSWORD or -password-file, never argv.
- Docs: README, security.md (+ accepted risk: encryption stays opt-in),
  architecture.md, progress.md, CHANGELOG.

Verified locally: panel-encrypted archive decrypts through the CLI and unpacks;
wrong password and password mismatch are refused; UI checked in a browser.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

cmd/selfpost-backup/main.go

@@ -9,21 +9,39 @@
 // Use -o to write to a file instead. The resulting archive contains DKIM private
 // keys, the admin password hash and SASL credentials — treat it as a secret
 // (spec 7.5.A).
+//
+// Given a password (SELFPOST_BACKUP_PASSWORD or -password-file, never an
+// argument, which would show up in the process list) the archive is written as
+// an encrypted .spbk envelope instead. Turn one back into a plain .tar.gz with
+// the same password:
+//
+//	docker exec -i <container> selfpost-backup -decrypt < backup.spbk > backup.tar.gz
 package main
 
 import (
 	"flag"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"codeberg.org/mix/selfpost/internal/backup"
 	"codeberg.org/mix/selfpost/internal/buildinfo"
+	"codeberg.org/mix/selfpost/internal/secretfile"
 )
 
+// passwordEnv names the environment variable holding the encryption password.
+// A password must never be a command-line argument: the process list is
+// readable by every process in the container.
+const passwordEnv = "SELFPOST_BACKUP_PASSWORD"
+
 func main() {
 	showVersion := flag.Bool("version", false, "print version and exit")
-	out := flag.String("o", "", "write the archive to this file instead of stdout")
+	out := flag.String("o", "", "write the output to this file instead of stdout")
+	in := flag.String("i", "", "read the encrypted archive from this file instead of stdin (-decrypt only)")
+	decrypt := flag.Bool("decrypt", false, "decrypt an encrypted backup (.spbk) back to a plain .tar.gz")
+	pwFile := flag.String("password-file", "", "read the encryption password from this file (first line); "+passwordEnv+" is used when unset")
 	flag.Parse()
 
 	if *showVersion {
@@ -31,40 +49,137 @@ func main() {
 		return
 	}
 
-	if err := run(*out); err != nil {
+	password, err := readPassword(*pwFile)
+	if err == nil {
+		if *decrypt {
+			err = runDecrypt(*in, *out, password)
+		} else {
+			err = run(*out, password)
+		}
+	}
+	if err != nil {
 		fmt.Fprintf(os.Stderr, "selfpost-backup: %v\n", err)
 		os.Exit(1)
 	}
 }
 
-func run(outPath string) error {
+// run writes a backup, encrypting it when a password was supplied.
+func run(outPath, password string) error {
 	dataDir := envDefault("SELFPOST_DATA_DIR", "/data")
 	dbPath := envDefault("SELFPOST_DB_PATH", filepath.Join(dataDir, "selfpost.db"))
 
-	w := os.Stdout
-	if outPath != "" {
-		// Backups are secret; create them owner-only.
-		f, err := os.OpenFile(outPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o600)
+	w, closeOut, err := openOutput(outPath)
+	if err != nil {
+		return err
+	}
+	defer closeOut()
+
+	sink := w
+	var env *secretfile.Writer
+	if password != "" {
+		env, err = secretfile.NewWriter(w, secretfile.TypeFullBackup, password)
 		if err != nil {
 			return err
 		}
-		defer f.Close()
-		w = f
+		sink = env
 	}
 
-	if err := backup.Create(w, backup.Params{
+	if err := backup.Create(sink, backup.Params{
 		DataDir: dataDir,
 		DBPath:  dbPath,
 		Version: buildinfo.Version,
 	}); err != nil {
 		return err
 	}
+	if env != nil {
+		if err := env.Close(); err != nil {
+			return err
+		}
+	}
 	if outPath != "" {
-		fmt.Fprintf(os.Stderr, "selfpost-backup: wrote %s (SelfPost %s)\n", outPath, buildinfo.Version)
+		kind := "plain"
+		if password != "" {
+			kind = "encrypted"
+		}
+		fmt.Fprintf(os.Stderr, "selfpost-backup: wrote %s (%s, SelfPost %s)\n", outPath, kind, buildinfo.Version)
 	}
 	return nil
 }
 
+// runDecrypt turns a .spbk envelope back into the plain gzip tar, so an
+// encrypted backup can be extracted with ordinary tools during a restore.
+func runDecrypt(inPath, outPath, password string) error {
+	if password == "" {
+		return fmt.Errorf("-decrypt needs the password (set %s or use -password-file)", passwordEnv)
+	}
+
+	r := io.Reader(os.Stdin)
+	if inPath != "" {
+		f, err := os.Open(inPath)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+		r = f
+	}
+
+	env, err := secretfile.NewReader(r, password)
+	if err != nil {
+		return err
+	}
+	if env.Type() != secretfile.TypeFullBackup {
+		return fmt.Errorf("that file is a %s, not a full backup", env.Type())
+	}
+
+	w, closeOut, err := openOutput(outPath)
+	if err != nil {
+		return err
+	}
+	defer closeOut()
+
+	if _, err := io.Copy(w, env); err != nil {
+		return err
+	}
+	if outPath != "" {
+		fmt.Fprintf(os.Stderr, "selfpost-backup: wrote %s (decrypted)\n", outPath)
+	}
+	return nil
+}
+
+// openOutput returns stdout, or a freshly created owner-only file: both plain
+// and encrypted backups are secret enough to keep off other users' eyes.
+func openOutput(outPath string) (io.Writer, func(), error) {
+	if outPath == "" {
+		return os.Stdout, func() {}, nil
+	}
+	f, err := os.OpenFile(outPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o600)
+	if err != nil {
+		return nil, nil, err
+	}
+	return f, func() { _ = f.Close() }, nil
+}
+
+// readPassword takes the password from the given file (first line) or, when no
+// file is named, from the environment. An empty result means "no encryption".
+func readPassword(pwFile string) (string, error) {
+	if pwFile == "" {
+		return os.Getenv(passwordEnv), nil
+	}
+	data, err := os.ReadFile(pwFile)
+	if err != nil {
+		return "", fmt.Errorf("read password file: %w", err)
+	}
+	// A password file is usually written with a trailing newline; take the first
+	// line and strip the line ending, but keep any other whitespace, which may
+	// well be part of the password.
+	line, _, _ := strings.Cut(string(data), "\n")
+	line = strings.TrimSuffix(line, "\r")
+	if line == "" {
+		return "", fmt.Errorf("password file %s is empty", pwFile)
+	}
+	return line, nil
+}
+
 func envDefault(key, def string) string {
 	if v := os.Getenv(key); v != "" {
 		return v