Persist Postfix queue and ship self-contained full backups.

Move the mail queue under /data so recreate no longer drops deferred mail, and archive data/, compose, .env, and certs/ together for restore on a fresh host.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/panel/main.go

@@ -78,6 +78,7 @@ type config struct {
 	saslDBPath string
 	saslRealm  string
 	postfixDir string
+	deployRoot string
 }
 
 func loadConfig() config {
@@ -138,6 +139,7 @@ func loadConfig() config {
 		saslDBPath: envDefault("SASL_DB_PATH", filepath.Join(dataDir, "sasl", "sasldb2")),
 		saslRealm:  saslRealm(),
 		postfixDir: envDefault("POSTFIX_DIR", filepath.Join(dataDir, "postfix")),
+		deployRoot: envDefault("SELFPOST_DEPLOY_ROOT", "/selfpost-deploy"),
 	}
 }