backup: the panel's full backup works, without the queue

Found by the e2e restore scenario of the next commit: the Full backup
button answered 200 with ten bytes. The archive includes the Postfix
queue, Postfix keeps every queue file and directory readable by its own
user alone (not configurable), and the panel runs as its own user - the
walk stopped at /data/postfix/queue/active with "permission denied"
after the headers were out. So since the queue moved under /data in
1.3.0. Confirmed on the test server: selfpost-backup as root makes the
whole archive, the same command as user panel fails the same way.

Owner, 2026-10-11: the panel's archive comes without the queue.

- backup.Params.Unprivileged: the queue is left out, and so is a file
  under certs/ that cannot be opened (the private key on the test
  server is root and Postfix's group only), named through OnWarn. The
  selfpost-backup command leaves it unset and takes both, as before.
- backup.Check reads through everything Create would archive, writing
  nothing. The panel runs it before the first byte: a failure is a
  message on the page, not a download cut short.
- The box, Help (design commit before this one) and the guide say what
  the panel's archive leaves out; the guide's comparison table has rows
  for the queue and the private key, and tells to put the key back
  before starting a restored instance.

  go vet ./internal/backup/ ./internal/web/handlers/: ok
  go test ./internal/backup/ ./cmd/... ./internal/web/...: ok
  make e2e on the test server, with the restore scenario: ok

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

internal/backup/backup_test.go

@@ -235,3 +235,47 @@ func TestCheckRestoreWrongFormatRejected(t *testing.T) {
 		t.Error("CheckRestore wrong format reported a restore")
 	}
 }
+
+// The panel's archive: no Postfix queue, everything else as before, and Check
+// agrees that it can be made.
+func TestCreateUnprivilegedLeavesTheQueueOut(t *testing.T) {
+	dataDir, dbPath, deployRoot := seedDataDir(t)
+	p := Params{DataDir: dataDir, DBPath: dbPath, Version: "1.2.3", DeployRoot: deployRoot, Unprivileged: true}
+
+	if err := Check(p); err != nil {
+		t.Fatalf("Check: %v", err)
+	}
+	var buf bytes.Buffer
+	if err := Create(&buf, p); err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	files := readArchive(t, buf.Bytes())
+	for name := range files {
+		if strings.HasPrefix(name, DataArchivePrefix+QueueDir) {
+			t.Errorf("the panel's archive holds %s", name)
+		}
+	}
+	for _, name := range []string{
+		DataArchivePrefix + "selfpost.db",
+		DataArchivePrefix + "opendkim/keys/example.com/selfpost.private",
+		DataArchivePrefix + "sasl/sasldb2",
+		DataArchivePrefix + "postfix/sender_login_maps",
+		ComposeFileName, EnvFileName,
+		CertsDirName + "/fullchain.pem",
+	} {
+		if _, ok := files[name]; !ok {
+			t.Errorf("archive missing %s", name)
+		}
+	}
+}
+
+// Check finds what Create would stumble over, before anything is written.
+func TestCheckReportsWhatCannotBeRead(t *testing.T) {
+	dataDir, dbPath, deployRoot := seedDataDir(t)
+	if err := os.Remove(filepath.Join(deployRoot, EnvFileName)); err != nil {
+		t.Fatal(err)
+	}
+	if err := Check(Params{DataDir: dataDir, DBPath: dbPath, Version: "1.2.3", DeployRoot: deployRoot}); err == nil {
+		t.Fatal("Check passed with .env missing from the deploy root")
+	}
+}