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>

build/entrypoint.sh

@@ -32,4 +32,23 @@ find /data/opendkim -type d -exec chmod 2750 {} +
 chmod 0640 /data/opendkim/KeyTable /data/opendkim/SigningTable
 find /data/opendkim/keys -type f -name '*.private' -exec chmod 0640 {} +
 
+# Application SASL accounts (spec 5.1, 9). The panel (user `panel`) writes the
+# sasldb2 via saslpasswd2; Postfix (user `postfix`) reads it to authenticate SMTP
+# clients. Share it through the `selfpost` group the same way as the DKIM tree:
+# setgid directory so new files inherit the group, and the database itself
+# group-readable (0640). Postfix wiring to actually consult it lands in Phase 5.
+mkdir -p /data/sasl
+chown -R panel:selfpost /data/sasl
+chmod 2750 /data/sasl
+[ -e /data/sasl/sasldb2 ] && chmod 0640 /data/sasl/sasldb2
+
+# Postfix sender_login_maps (spec 5.1). The panel writes it; Postfix reads it.
+# Ensure the file exists (empty is fine) before Postfix starts so a reload that
+# references it never fails on a missing file, and keep it group-readable.
+mkdir -p /data/postfix
+[ -e /data/postfix/sender_login_maps ] || : > /data/postfix/sender_login_maps
+chown -R panel:selfpost /data/postfix
+chmod 2750 /data/postfix
+chmod 0640 /data/postfix/sender_login_maps
+
 exec /usr/bin/supervisord -c /etc/supervisor/supervisord.conf