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>

cmd/panel/httpserver.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"codeberg.org/mix/selfpost/internal/app"
+	"codeberg.org/mix/selfpost/internal/buildinfo"
 	"codeberg.org/mix/selfpost/internal/domain"
 	"codeberg.org/mix/selfpost/internal/postfix"
 	"codeberg.org/mix/selfpost/internal/store"
@@ -28,6 +29,9 @@ func serveHTTP(ctx context.Context, cfg config, st *store.Store) error {
 		Hostname:     cfg.hostname,
 		CookieSecure: cfg.cookieSecure,
 		MailLogPath:  cfg.mailLog,
+		DataDir:      cfg.dataDir,
+		DBPath:       cfg.dbPath,
+		Version:      buildinfo.Version,
 	}, cfg.setupTokenPath)
 	if err != nil {
 		return err