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>

cmd/panel/main.go

@@ -53,6 +53,10 @@ type config struct {
 
 	opendkimDir     string
 	dkimSelectorDef string
+
+	saslDBPath string
+	saslRealm  string
+	postfixDir string
 }
 
 func loadConfig() config {
@@ -74,9 +78,30 @@ func loadConfig() config {
 		// entrypoint.sh prepares (setgid, shared `selfpost` group).
 		opendkimDir:     envDefault("OPENDKIM_DIR", filepath.Join(dataDir, "opendkim")),
 		dkimSelectorDef: envDefault("DKIM_SELECTOR_DEFAULT", "selfpost"),
+
+		// Application SASL accounts and the Postfix sender map (spec 5.1, 9),
+		// both under /data so they survive restarts. The SASL realm defaults to
+		// the server hostname so account identities line up with Postfix's SASL
+		// configuration; it falls back to localhost outside the container.
+		saslDBPath: envDefault("SASL_DB_PATH", filepath.Join(dataDir, "sasl", "sasldb2")),
+		saslRealm:  saslRealm(),
+		postfixDir: envDefault("POSTFIX_DIR", filepath.Join(dataDir, "postfix")),
 	}
 }
 
+// saslRealm chooses the realm new SASL accounts live under. It mirrors the
+// hostname Postfix's SASL layer uses so a client authenticating with a bare
+// login resolves to the right account (finalised in Phase 5).
+func saslRealm() string {
+	if r := os.Getenv("SASL_REALM"); r != "" {
+		return r
+	}
+	if h := os.Getenv("SELFPOST_HOSTNAME"); h != "" {
+		return h
+	}
+	return "localhost"
+}
+
 func envDefault(key, def string) string {
 	if v := os.Getenv(key); v != "" {
 		return v