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/password.go

@@ -0,0 +1,27 @@
+package app
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+)
+
+// passwordBytes is the number of random bytes behind a generated application
+// password. 24 bytes = 192 bits of entropy, well above any brute-force concern
+// for a SASL credential the panel shows exactly once (spec 7.6.1).
+const passwordBytes = 24
+
+// generatePassword returns a strong, URL-safe random password for an
+// application's SASL account. The panel generates it, shows it once and never
+// stores the plaintext (spec 7.6.1); sasldb2 keeps only the hashed form.
+//
+// base64url output keeps the password to a safe ASCII alphabet with no shell or
+// SMTP-special characters, so it survives being typed into client configuration
+// and passed to saslpasswd2 over stdin unchanged.
+func generatePassword() (string, error) {
+	buf := make([]byte, passwordBytes)
+	if _, err := rand.Read(buf); err != nil {
+		return "", fmt.Errorf("generate password: %w", err)
+	}
+	return base64.RawURLEncoding.EncodeToString(buf), nil
+}