feat: replace tar+DB+LND backup with ext4+rsync workflow

- Rewrite sovran-hub-backup.sh: rsync-based mirror to stable current/
  path, ext4 validation, no tar/pg_dump/mariadb-dump/LND orchestration,
  exit-24 nonfatal for /home, INCOMPLETE/BACKUP_COMPLETE markers, flock
- Update server.py: _is_supported_backup_fstype accepts only ext4
- Update support.js: require ext4, explain database limitations, update
  failure message from exFAT to ext4
- Update sovran-hub.nix: replace pkgs.gnutar with pkgs.rsync + pkgs.acl
- Rewrite test_manual_backup_workflow.py: 40 new tests covering rsync
  options, ext4 acceptance, exit-24 handling, no-delete, stable current/
  path, INCOMPLETE markers, behavioral rsync tests, and regressions

modules/core/sovran-hub.nix

@@ -393,7 +393,8 @@ in
         pkgs.coreutils
         pkgs.findutils
         pkgs.gnugrep
-        pkgs.gnutar
+        pkgs.rsync
+        pkgs.acl
         pkgs.util-linux
       ]
       ++ lib.optional cfg.services.bitcoin config.services.bitcoind.package