audit 3: backup CLI, envelope budget, directory sync, docs; findings of the reviews

Steps 3-6 of stage 3 and what the two reviews found in steps 1-6, in one
commit: the review fixes sit in the same hunks.

Step 3, CLI backup (M3, M5, L1, L2; L4 of the stage's list):
- backup.snapshotDB refuses a database that is not there and opens the
  source mode=ro (measured: mode=ro creates no file).
- selfpost-backup holds an encrypting password to
  validate.MinSecretFilePasswordLen; -o of both directions writes a
  temporary file beside the name and renames it whole; -decrypt peeks the
  first chunk (password and type authenticated) before the output exists.
- what a backup leaves out (a link under data/, a link under certs/ to a
  directory, a socket...) is told to OnWarn; certs/ with no file is
  backup.ErrNoCerts, refused by Check and Create, named by the panel.
- parseTrustedProxies: a bare address takes /128 when written with a
  colon (::ffff:10.0.0.1/32 was ::/32).
Step 4, secretfile (M4, L3): scrypt's V at most 128 MiB, r <= 64, p <= 2;
bytes after the last chunk are ErrCorrupt.
Step 5 (L7): atomicfile.SyncDir after the rename, used by Write,
replaceSASLDB and the CLI; atomicfile_test.go.
Step 6: security.md and guide on backup and restore.

Reviews (technical, security):
- H1: a restore of a server with DMARC on was refused on every start - the
  archive records no owner, tar as root left the DMARC maps root's, and the
  entrypoint only chowned the maps it named. It now hands every file beside
  queue/ in /data/postfix to panel:selfpost 0640; e2e writes an extra map
  before the backup and finds it panel:selfpost 640 after a root unpack.
- the comparison reads regular files only (Lstat, size, LimitReader); a
  record unpacked again wins over the earlier check's mark; ConsumeRestore
  removes both; the mark is synced; a journal file beside a freshly unpacked
  database is refused.
- AttachRefusalLine refuses an empty line as the old statement did; skipped
  entries named in words; -decrypt of an empty payload is no failure; guide
  wording on the shell's '>' and a killed CLI; the in-place restore's
  'mv .' (older than this stage) fixed.

Tested on the test server: go vet + go test of 25 packages, make e2e (all
pass, 643 s) on this tree.

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

cmd/selfpost-backup/main_test.go

@@ -162,3 +162,76 @@ func TestReadPassword(t *testing.T) {
 		t.Error("-decrypt without a password was accepted")
 	}
 }
+
+// An encrypted backup is held to the panel's password floor (audit
+// 2026-10-12, M5); a refused one writes nothing.
+func TestEncryptionPasswordFloor(t *testing.T) {
+	seedProject(t)
+	out := filepath.Join(t.TempDir(), "backup.spbk")
+	err := run(out, "eleven-char")
+	if err == nil || !strings.Contains(err.Error(), "at least 12 characters") {
+		t.Fatalf("an 11-character password = %v, want refused", err)
+	}
+	if _, statErr := os.Stat(out); !os.IsNotExist(statErr) {
+		t.Errorf("the refused backup wrote %s", out)
+	}
+}
+
+// A wrong SELFPOST_DB_PATH makes no backup and no database (audit
+// 2026-10-12, M3).
+func TestBackupOfAMissingDatabaseIsRefused(t *testing.T) {
+	_, dataDir := seedProject(t)
+	missing := filepath.Join(dataDir, "typo.db")
+	t.Setenv("SELFPOST_DB_PATH", missing)
+	out := filepath.Join(t.TempDir(), "backup.tar.gz")
+	if err := run(out, ""); err == nil {
+		t.Fatal("a backup of a database that is not there was made")
+	}
+	for _, p := range []string{missing, out} {
+		if _, err := os.Stat(p); !os.IsNotExist(err) {
+			t.Errorf("the refused backup left %s (stat err = %v)", p, err)
+		}
+	}
+}
+
+// -decrypt -o writes its file whole or not at all: a wrong password or an
+// envelope cut short leaves the file that had the name before, and nothing
+// else in its directory (audit 2026-10-12, L1 and stage 3).
+func TestDecryptLeavesTheOutputAloneOnFailure(t *testing.T) {
+	seedProject(t)
+	dir := t.TempDir()
+	encrypted := filepath.Join(dir, "backup.spbk")
+	const password = "a long enough password"
+	if err := run(encrypted, password); err != nil {
+		t.Fatalf("create encrypted backup: %v", err)
+	}
+	sealed, err := os.ReadFile(encrypted)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cut := filepath.Join(dir, "cut.spbk")
+	writeFile(t, cut, string(sealed[:len(sealed)-100]))
+
+	outDir := t.TempDir()
+	out := filepath.Join(outDir, "backup.tar.gz")
+	const before = "the archive that was here"
+	for _, c := range []struct{ name, in, password, want string }{
+		{"wrong password", encrypted, "the wrong password", "wrong password"},
+		{"cut short", cut, password, "corrupted"},
+	} {
+		t.Run(c.name, func(t *testing.T) {
+			writeFile(t, out, before)
+			err := runDecrypt(c.in, out, c.password)
+			if err == nil || !strings.Contains(err.Error(), c.want) {
+				t.Fatalf("runDecrypt = %v, want %q", err, c.want)
+			}
+			if got, _ := os.ReadFile(out); string(got) != before {
+				t.Errorf("the failed decryption changed %s to %d bytes", out, len(got))
+			}
+			entries, _ := os.ReadDir(outDir)
+			if len(entries) != 1 {
+				t.Errorf("the failed decryption left files beside its output: %v", entries)
+			}
+		})
+	}
+}