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/postfix/postfix_test.go

@@ -0,0 +1,118 @@
+package postfix
+
+import (
+	"os"
+	"testing"
+)
+
+func TestRenderSenderLoginMaps(t *testing.T) {
+	// Deliberately unsorted, with two logins sharing one wildcard key
+	// (many-to-one, spec 5.1 §4) to exercise merge + sort.
+	bindings := []Binding{
+		{"@zeta.example", "z1"},
+		{"alerts@alpha.example", "a-listed"},
+		{"@alpha.example", "a2"},
+		{"@alpha.example", "a1"},
+	}
+	got, err := renderSenderLoginMaps(bindings)
+	if err != nil {
+		t.Fatalf("renderSenderLoginMaps: %v", err)
+	}
+	want := "@alpha.example a1,a2\n" +
+		"@zeta.example z1\n" +
+		"alerts@alpha.example a-listed\n"
+	if string(got) != want {
+		t.Errorf("map =\n%q\nwant\n%q", got, want)
+	}
+}
+
+func TestRenderSenderLoginMapsEmpty(t *testing.T) {
+	got, err := renderSenderLoginMaps(nil)
+	if err != nil {
+		t.Fatalf("renderSenderLoginMaps(nil): %v", err)
+	}
+	if len(got) != 0 {
+		t.Errorf("expected empty map, got %q", got)
+	}
+}
+
+func TestRenderSenderLoginMapsDedupesLogin(t *testing.T) {
+	bindings := []Binding{
+		{"@a.example", "dup"},
+		{"@a.example", "dup"},
+	}
+	got, err := renderSenderLoginMaps(bindings)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "@a.example dup\n" {
+		t.Errorf("map = %q, want single deduped login", got)
+	}
+}
+
+func TestAssertMapSafeRejectsInjection(t *testing.T) {
+	bad := []struct{ addr, login string }{
+		{"@exa mple.com", "log"},
+		{"@example.com\nx y z", "log"},
+		{"@example.com", "log,evil"},
+		{"@example.com", "log in"},
+		{"@example.com", "log@realm"}, // '@' would confuse sasldb realm handling
+		{"", "log"},
+		{"@example.com", ""},
+	}
+	for _, b := range bad {
+		if err := assertMapSafe(b.addr, b.login); err == nil {
+			t.Errorf("assertMapSafe(%q,%q) = nil, want error", b.addr, b.login)
+		}
+	}
+	if err := assertMapSafe("alerts@example.com", "app_1-x"); err != nil {
+		t.Errorf("assertMapSafe of a clean pair errored: %v", err)
+	}
+}
+
+func newTestPostfix(t *testing.T) (*Postfix, *int) {
+	t.Helper()
+	dir := t.TempDir()
+	reloads := 0
+	p := New(dir)
+	p.reload = func() error { reloads++; return nil }
+	return p, &reloads
+}
+
+func TestRebuildSenderLoginMapsWritesAndReloads(t *testing.T) {
+	p, reloads := newTestPostfix(t)
+	if err := p.RebuildSenderLoginMaps([]Binding{{"@example.com", "app1"}}); err != nil {
+		t.Fatalf("RebuildSenderLoginMaps: %v", err)
+	}
+	if *reloads != 1 {
+		t.Errorf("reload called %d times, want 1", *reloads)
+	}
+	data, err := os.ReadFile(p.senderLoginMapsPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != "@example.com app1\n" {
+		t.Errorf("map file = %q", data)
+	}
+}
+
+func TestRebuildRejectsUnsafeWithoutWriting(t *testing.T) {
+	p, reloads := newTestPostfix(t)
+	// Seed a known-good file so we can prove the failed rebuild left it untouched.
+	if err := p.RebuildSenderLoginMaps([]Binding{{"@good.example", "ok"}}); err != nil {
+		t.Fatal(err)
+	}
+	before, _ := os.ReadFile(p.senderLoginMapsPath)
+
+	err := p.RebuildSenderLoginMaps([]Binding{{"@bad.example", "evil\nlogin"}})
+	if err == nil {
+		t.Fatal("expected rebuild to reject unsafe login")
+	}
+	after, _ := os.ReadFile(p.senderLoginMapsPath)
+	if string(after) != string(before) {
+		t.Errorf("map file changed on failed rebuild: %q", after)
+	}
+	if *reloads != 1 {
+		t.Errorf("reload called %d times, want 1 (no reload on failure)", *reloads)
+	}
+}