Phase 4: applications + SASL (sasldb2) + sender_login_maps

Adds application accounts bound to domains: a SASL login/password in
sasldb2, a per-application address mode (wildcard @domain or an explicit
list), and matching smtpd_sender_login_maps bindings — with create,
list, edit-mode, delete and password regeneration (spec 4.1, 5.1,
7.2.5-9). Generated passwords are shown exactly once and never stored in
plaintext (7.6.1).

- internal/store/applications.go: transactional CRUD; globally unique
  login; ListBindings (address->login) as the map source; logins-by-
  domain for pre-cascade SASL cleanup.
- internal/app: saslpasswd2 wrapper (password via stdin, login as a
  whitelisted argv element, no shell — 7.6.3); strong base64url password;
  address validation that enforces domain ownership before any config
  write (7.6.2); service orchestrating store + sasldb2 + map with full
  rollback on partial failure.
- internal/postfix: sender_login_maps regenerated as a pure function of
  the registry (many-to-one logins merged per address), atomic write,
  injection backstop (7.6.4).
- Postfix reload, corrected: `postfix start-fg` forks a separate master,
  so signalling the supervised process never reaches it. Reload now runs
  the canonical `postfix reload` via a one-shot supervisord program the
  unprivileged panel triggers over the group control socket. Verified in
  mail.log.
- domain.Service.Delete purges the domain's SASL accounts, then cascades,
  then rebuilds the sender map and reloads; manual reload now covers both
  OpenDKIM and Postfix.
- web: application management in the domain page, one-time credential
  shown inline; postfix joins the selfpost group and entrypoint normalises
  /data/sasl and /data/postfix (setgid, group-readable) with self-heal.

Verified on the dev server: gofmt/vet/test green, image builds, and a
container e2e covers the full application lifecycle, domain-delete
cascade, restart persistence, and a real postfix reload.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/app/sasl_test.go

@@ -0,0 +1,76 @@
+package app
+
+import (
+	"strings"
+	"testing"
+)
+
+type fakeRun struct {
+	args  []string
+	stdin string
+	calls int
+}
+
+func newFakeSASL() (*SASLDB, *fakeRun) {
+	fr := &fakeRun{}
+	s := NewSASLDB("/data/sasl/sasldb2", "mail.example.com")
+	s.run = func(args []string, stdin []byte) error {
+		fr.calls++
+		fr.args = args
+		fr.stdin = string(stdin)
+		return nil
+	}
+	return s, fr
+}
+
+func TestSASLSetPassesPasswordOnStdinNotArgv(t *testing.T) {
+	s, fr := newFakeSASL()
+	const secret = "s3cr3t-p4ss"
+	if err := s.Set("alerts", secret); err != nil {
+		t.Fatalf("Set: %v", err)
+	}
+	if fr.stdin != secret {
+		t.Errorf("password not passed on stdin: got %q", fr.stdin)
+	}
+	joined := strings.Join(fr.args, " ")
+	if strings.Contains(joined, secret) {
+		t.Errorf("password leaked into argv: %q", joined)
+	}
+	// Expected fixed flags and the login as its own trailing argument.
+	want := []string{"-p", "-c", "-f", "/data/sasl/sasldb2", "-u", "mail.example.com", "alerts"}
+	if len(fr.args) != len(want) {
+		t.Fatalf("args = %v, want %v", fr.args, want)
+	}
+	for i := range want {
+		if fr.args[i] != want[i] {
+			t.Fatalf("args = %v, want %v", fr.args, want)
+		}
+	}
+}
+
+func TestSASLDeleteArgs(t *testing.T) {
+	s, fr := newFakeSASL()
+	if err := s.Delete("alerts"); err != nil {
+		t.Fatalf("Delete: %v", err)
+	}
+	want := []string{"-d", "-f", "/data/sasl/sasldb2", "-u", "mail.example.com", "alerts"}
+	if strings.Join(fr.args, " ") != strings.Join(want, " ") {
+		t.Errorf("delete args = %v, want %v", fr.args, want)
+	}
+	if fr.stdin != "" {
+		t.Errorf("delete should not send stdin, got %q", fr.stdin)
+	}
+}
+
+func TestSASLRejectsInvalidLoginBeforeExec(t *testing.T) {
+	s, fr := newFakeSASL()
+	if err := s.Set("bad login", "pw"); err == nil {
+		t.Error("Set accepted invalid login")
+	}
+	if err := s.Delete("bad@login"); err == nil {
+		t.Error("Delete accepted invalid login")
+	}
+	if fr.calls != 0 {
+		t.Errorf("saslpasswd2 invoked %d times for invalid logins, want 0", fr.calls)
+	}
+}