Phase 9: full backup/restore + domain export/import (spec 7.5, 11.6)

Full server backup (spec 7.5.A): internal/backup produces a tar.gz of all of
/data — a consistent SQLite snapshot via VACUUM INTO, DKIM keys, sasldb2 and a
version manifest; TLS certs (tls/) and the Postfix queue are excluded. Two equal
paths: the panel button (POST /backup, no-store) and the selfpost-backup CLI via
docker exec (spec 11.6). CheckRestore runs before store.Open: a manifest version
mismatch refuses to boot with the image tag to use; a match consumes the
manifest so it only guards the first post-restore boot. Restore is not a
separate branch — Postfix/OpenDKIM regenerate from the restored SQLite as on any
start.

Domain export/import (spec 7.5.B): DomainExport carries the DKIM private key and
each application's working password. SASL secrets are read from sasldb2 via
db_dump (the userPassword property is plaintext) and, on import, re-keyed under
the local realm with saslpasswd2 — so credentials keep working on an instance
with a different hostname, with no DKIM DNS change. Import validates and rolls
back atomically on any failure. db-util (db_dump) is now an explicit image dep.

Verified on the server (selfpost:p9): gofmt/vet/test green; container e2e for
cross-realm domain export/import (SMTP AUTH 235 under the new realm), CLI and
panel backups, same-version restore, and version-mismatch refusal.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/domain/service.go

@@ -18,6 +18,12 @@ type Applications interface {
 	// Resync rebuilds smtpd_sender_login_maps from the remaining applications
 	// and reloads Postfix.
 	Resync() error
+	// Secret returns an application's stored password, for a domain export
+	// (spec 7.5.B).
+	Secret(login string) (string, error)
+	// ImportApplication re-creates an application (registry row + SASL account)
+	// from a domain-export file, without rebuilding the sender map (spec 7.5.B).
+	ImportApplication(domainID int64, login, mode string, addresses []string, password string) error
 }
 
 // Service coordinates the places a sending domain lives: the SQLite registry,