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/web/web.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"time"
 
+	"codeberg.org/mix/selfpost/internal/app"
 	"codeberg.org/mix/selfpost/internal/domain"
 	"codeberg.org/mix/selfpost/internal/store"
 )
@@ -32,6 +33,7 @@ type Config struct {
 type Server struct {
 	store    *store.Store
 	domains  *domain.Service
+	apps     *app.Service
 	cfg      Config
 	tmpl     *templates
 	sessions *sessionStore
@@ -43,8 +45,9 @@ type Server struct {
 
 // New builds the panel server. setupTokenPath is where the current setup token
 // is mirrored on disk (spec 7.6.1); domains is the sending-domain service that
-// owns DKIM keys and the OpenDKIM tables (spec 6).
-func New(st *store.Store, domains *domain.Service, cfg Config, setupTokenPath string) (*Server, error) {
+// owns DKIM keys and the OpenDKIM tables (spec 6); apps owns application SASL
+// accounts and the Postfix sender map (spec 5.1).
+func New(st *store.Store, domains *domain.Service, apps *app.Service, cfg Config, setupTokenPath string) (*Server, error) {
 	tmpl, err := loadTemplates()
 	if err != nil {
 		return nil, err
@@ -52,6 +55,7 @@ func New(st *store.Store, domains *domain.Service, cfg Config, setupTokenPath st
 	s := &Server{
 		store:    st,
 		domains:  domains,
+		apps:     apps,
 		cfg:      cfg,
 		tmpl:     tmpl,
 		sessions: newSessionStore(),
@@ -97,6 +101,10 @@ func (s *Server) Handler() http.Handler {
 	authed.HandleFunc("GET /domains/{id}", s.handleDomainDetail)
 	authed.HandleFunc("GET /domains/{id}/delete", s.handleDeleteConfirm)
 	authed.HandleFunc("POST /domains/{id}/delete", s.handleDeleteDomain)
+	authed.HandleFunc("POST /domains/{id}/applications", s.handleAddApplication)
+	authed.HandleFunc("POST /applications/{aid}/mode", s.handleUpdateAppMode)
+	authed.HandleFunc("POST /applications/{aid}/password", s.handleRegenPassword)
+	authed.HandleFunc("POST /applications/{aid}/delete", s.handleDeleteApplication)
 	authed.HandleFunc("POST /reload", s.handleReload)
 	mux.Handle("/", s.requireAuth(authed))