[Feature]: Light backup mode to exclude node_user_usages/node_usages rows (default on) #43

Problem

On large PasarGuard installations, node_user_usages and node_usages dominate both backup size and restore time. On one production panel node_user_usages had ~247M rows; a normal backup is hundreds of MB and grows forever. Most operators only need accounts, groups, hosts, nodes and settings for disaster recovery, not the full per-node traffic history.

--ignore-table alone is not enough because it also drops the CREATE TABLE, so the restored panel is missing the usage tables and the usage jobs fail.

Proposal

Add a first-class light backup mode to pasarguard backup-service:

• Exclude only the rows of node_user_usages and node_usages.
• Keep their schema (CREATE TABLE, indexes, foreign keys) so the panel works after restore.
• Show it in the menu as an option (e.g. 7. Toggle light backup [ON/OFF]).
• Enable it by default for new installs and keep the choice across script updates, with a clear opt-out to a full backup.
• Make it work for the standard MySQL/MariaDB paths (and custom DB mode if possible).

Implementation sketch

1. Add --ignore-table=${db_name}.node_user_usages --ignore-table=${db_name}.node_usages to the MySQL/MariaDB dump invocations.
2. After the data dump, append a schema-only dump for those two tables:
   mariadb-dump --no-data --skip-add-drop-table --skip-comments --compact --no-create-db "$db" node_user_usages node_usages
   then wrap CREATE TABLE with CREATE TABLE IF NOT EXISTS, remove the old -- Dump completed on ... footer and append a fresh one so the restore validator still passes.
3. Store the state in a persistent flag (e.g. /etc/pasarguard/backup-light), create it by default, and have both backup_service and backup_command read it.
4. Document that restoring over an existing database keeps old usage rows (the dump intentionally excludes them); restoring into a fresh/dropped database is the supported path.

This should compose with PR #40 (--single-transaction for non-locking MySQL dumps), which also touches the same 7 dump calls.

Testing / reference

I built and tested a standalone installer/patch for PasarGuard 5.5.1 on a test panel:
https://github.com/Aknuun/pasarguard-light-backup

• Light archive: 0 INSERT statements for both usage tables, CREATE TABLE for both, valid completion marker.
• DROP DATABASE pasarguard + restore succeeded: panel 200, node connected, 2 users / 9 hosts restored, usage tables empty.
• Existing test archive was ~63 KB instead of ~852 KB full, mostly because the two usage tables were excluded.

Happy to open a PR against lib/pasarguard-backup.sh if maintainers want to take this path. If the default-on choice is controversial, at minimum exposing the toggle would already help large installations.