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/httpserver.go

@@ -7,7 +7,9 @@ import (
 	"net/http"
 	"time"
 
+	"codeberg.org/mix/selfpost/internal/app"
 	"codeberg.org/mix/selfpost/internal/domain"
+	"codeberg.org/mix/selfpost/internal/postfix"
 	"codeberg.org/mix/selfpost/internal/store"
 	"codeberg.org/mix/selfpost/internal/web"
 )
@@ -22,9 +24,13 @@ func serveHTTP(ctx context.Context, cfg config) error {
 	}
 	defer st.Close()
 
-	domains := domain.NewService(st, domain.NewOpenDKIM(cfg.opendkimDir), cfg.dkimSelectorDef)
+	// Applications own the SASL accounts and the Postfix sender map; the domain
+	// service delegates to them when a domain (and its applications) is deleted.
+	pf := postfix.New(cfg.postfixDir)
+	apps := app.NewService(st, app.NewSASLDB(cfg.saslDBPath, cfg.saslRealm), pf)
+	domains := domain.NewService(st, domain.NewOpenDKIM(cfg.opendkimDir), apps, cfg.dkimSelectorDef)
 
-	srvApp, err := web.New(st, domains, web.Config{
+	srvApp, err := web.New(st, domains, apps, web.Config{
 		Hostname:     cfg.hostname,
 		CookieSecure: cfg.cookieSecure,
 	}, cfg.setupTokenPath)